Maturitas 82 (2015) 245–255
Contents lists available at ScienceDirect
Maturitas
journal homepage: www.elsevier.com/locate/maturitas
Review article
Bone biology, signaling pathways, and therapeutic targets for
osteoporosis
Nicole M. Iñiguez-Ariza ∗ , Bart L. Clarke ∗
Mayo Clinic E18-A, 200 1st Street SW, Rochester, Minnesota 55905, USA
a r t i c l e
i n f o
Article history:
Received 1 July 2015
Accepted 6 July 2015
Keywords:
Osteoporosis
Cathepsin K inhibitors
Anti-sclerostin antibody
Anti-Dickkopf antibody
PTH
PTHrP
a b s t r a c t
Major advances have occurred recently in the treatment of osteoporosis in recent years. Most patients
are currently treated with bisphosphonates, denosumab, raloxifene, or teriparatide, and in some countries, strontium ranelate. Strontium ranelate and calcitonin have recently had their use restricted due
to cardiovascular concerns and malignancy, respectively. The available agents have generally provided
excellent options that effectively reduce fracture risk. New targets are being sought based on appreciation of the bone biology and signaling pathways involved in bone formation and resorption. These agents
will directly target these signaling pathways, and further expand the options available for treatment of
osteoporosis.
© 2015 Elsevier Ireland Ltd. All rights reserved.
Contents
1.
2.
3.
4.
5.
6.
7.
8.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 246
Bone biology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 246
2.1.
Bone modeling and remodeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 246
2.2.
Osteoclasts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247
2.3.
Osteoblasts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247
2.4.
Bone loss . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 248
Signaling pathways . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 248
Antiresorptive pathways . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 248
4.1.
Bisphosphonates and denosumab . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 248
4.2.
Cathepsin K . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 248
Anabolic pathways . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 248
5.1.
PTH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 248
5.2.
PTHrP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 249
5.3.
Wnt signaling pathway . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 249
5.4.
Activin A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 249
5.5.
Bone morphogenetic proteins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 249
5.6.
Nitroglycerin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 249
Therapeutic targets for osteoporosis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 249
Antiresorptive agents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 249
7.1.
Cathepsin K inhibitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 249
Anabolic agents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 251
8.1.
PTH analogues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 251
8.2.
PTH-related protein analogues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 251
∗ Corresponding authors. Fax: +1 507 284 5745.
E-mail addresses: [email protected] (N.M. Iñiguez-Ariza),
[email protected] (B.L. Clarke).
http://dx.doi.org/10.1016/j.maturitas.2015.07.003
0378-5122/© 2015 Elsevier Ireland Ltd. All rights reserved.
246
N.M. Iñiguez-Ariza, B.L. Clarke / Maturitas 82 (2015) 245–255
8.3.
Activators of the Wnt/␤-catenin signaling pathway . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 252
8.4.
Other anabolic agents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253
9.
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253
Practice points . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253
Research agenda . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253
Conflict of interest . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253
Authors’ contribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253
Funding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253
1. Introduction
Discovery of new therapeutic targets for treatment of osteoporosis is important because of the limited efficacy and variable
toxicities of currently approved agents. These agents, while significantly more effective than estrogen, have a relatively moderate
ability to reduce hip, vertebral, and non-vertebral fractures. Current treatments reduce vertebral fracture risk significantly more
than hip or nonvertebral fracture risk.
Currently approved agents in the U.S. include bisphosphonates,
hormone therapy, raloxifene, calcitonin, teriparatide, or denosumab. Calcitonin was recently removed from the market in several
countries due to concerns regarding cancer risk with an oral formulation used in clinical trials, and the FDA recently recommended
against long-term treatment of osteoporosis with calcitonin. Teriparatide (PTH 1–34) and PTH 1–84 remain the only approved agents
for stimulating new bone formation in some countries.
This narrative review will briefly summarize the bone biology
relevant for understanding osteoporosis, describe the critical signaling pathways involved in bone resorption and bone formation
and discuss potential therapeutic targets, and review data available
for some of the newer agents developed against these targets.
2. Bone biology
2.1. Bone modeling and remodeling
The skeleton undergoes longitudinal and radial growth, modeling, and remodeling during life. Longitudinal and radial growth
occurs during childhood and adolescence. Growth plates cause longitudinal growth due to cartilage proliferation in the epiphyseal
and metaphyseal regions of long bones. Newly-produced cartilage
mineralizes to form primary new bone. Radial growth occurs due
to periosteal apposition during and after puberty, and also later in
life with normal aging.
Modeling results in bones changing shape due to mechanical
forces exerted on the skeleton, which leads to gradual adjustment
in the shape of bone due to the physical forces encountered. Bones
widen or change their axis by the removal or addition of bone to
the appropriate surfaces by the action of osteoblasts and osteoclasts in response to biomechanical forces. Bones normally widen
with aging in response to periosteal apposition of new bone and
endosteal resorption of old bone. Bone formation and resorption
are less tightly coupled during modeling than during remodeling.
Bone modeling occurs less frequently than remodeling in adults
[1]. Modeling increases in hypoparathyroidism [2], chronic kidney
disease [3], or treatment with anabolic agents such as teriparatide
[4].
Bone remodeling occurs throughout life in order to renew bone,
and to maintain bone strength and preserve mineral homeostasis.
Remodeling removes discrete packets of old bone continuously,
and replaces these with packets of newly synthesized proteinaceous matrix which subsequently mineralize to form new
bone. Remodeling thereby prevents the accumulation of bone
microdamage that could lead to fracture.
The bone remodeling unit is a tightly-coupled group of osteoclasts and osteoblasts that sequentially resorbs old bone and forms
new bone. Bone remodeling activity increases around the time of
menopause, and continues at an accelerated rate for the first 5–10
years after menopause. After this period of rapid activity, bone
remodeling slows down, but continues at a faster rate than in premenopausal women. Bone remodeling also increases mildly with
aging in men.
The remodeling cycle has four sequential phases. Recruitment
and activation of bone resorbing osteoclasts occurs first, followed
by bone resorption for several weeks. Bone resorption then stops,
with subsequent reversal, followed by an extended period of bone
formation. Remodeling sites may develop randomly at a low rate,
but seem to concentrate in areas of microdamage requiring repair
[5,6].
Activation of bone resorption involves recruitment and activation of mononuclear monocyte–macrophage osteoclast precursors
from the circulation [7], lifting of the endosteal membrane containing lining cells off the bone surface, and fusion of multiple
mononuclear cells to form multinucleated preosteoclasts on the
surface of the exposed bone. Preosteoclasts bind to the bone matrix
via interactions between integrin receptors in their cell membranes
and RGD (arginine, glycine, and asparagine)-containing peptides in
matrix proteins, and form sealing zones around the bone-resorbing
compartments beneath multinucleated osteoclasts.
Osteoclast-mediated bone resorption takes about 2–4 weeks
during each remodeling cycle. Osteoclast formation, activation, and
resorption are regulated by the ratio of receptor activator of nuclear
factor ␬B ligand (RANKL) to osteoprotegerin (OPG), and exposure of preosteoclasts to interleukin (IL)-1 and IL-6, macrophage
colony stimulating factor (M-CSF), parathyroid hormone (PTH),
1,25-dihydroxyvitamin D, and calcitonin [8,9]. Activated multinucleated osteoclasts secrete hydrogen ions via H+ -ATPase proton
pumps and chloride channels within their ruffled border cell membranes into the resorption pits beneath them to lower the pH
within the bone-resorbing compartment to as low as 4.5, which
helps solubilize bone mineral [10]. Resorbing osteoclasts secrete
tartrate-resistant acid phosphatase (TRAP), cathepsin K, matrix
metalloproteinase-9 (MMP-9), and gelatinase from cytoplasmic
lysosomes [11] to digest organic matrix, resulting in formation of
saucer-shaped Howship’s lacunae on the surface of trabecular bone
and resorption tunnels in cortical bone. The resorption phase is
completed when the multinucleated osteoclasts stop functioning
and undergo apoptosis [12,13].
During the reversal phase, bone resorption converts to bone
formation. At the completion of bone resorption, resorption
cavities contain several types of mononuclear cells, including
monocytes, osteocytes released from resorbed bone matrix, and
pre-osteoblasts recruited to begin new bone formation. Coupling
signals linking completion of bone resorption to initiation of
bone formation remain an area of active investigation. Coupling
signal candidates include bone matrix-derived factors such as
N.M. Iñiguez-Ariza, B.L. Clarke / Maturitas 82 (2015) 245–255
transforming growth factor (TGF)-␤, insulin-like growth factor
(IGF)-1, IGF-2, bone morphogenetic proteins (BMPs), plateletderived growth factors (PDGFs), and fibroblast growth factors
(FGFs) [14–16]. TGF-␤ released from bone matrix decreases
osteoclast bone resorption by inhibiting RANKL production by
osteoblasts. Osteoclasts also secrete ephrins and other factors
that directly influence activation of osteoblast precursors and
osteoblasts [17]. Multiple other molecules likely also play a role in
recruiting osteoblast precursors to resorption lacunae.
The reversal phase may also be mediated by the strain gradient
in the cutting cone in the cortical resorption tunnel or Howship’s
lacunae [18,19]. As osteoclasts resorb cortical bone in a cutting cone
within a cortical resorption tunnel, strain is reduced in front and
increased behind, and in Howship’s lacunae, strain is highest at the
base and less in surrounding bone at the edges of the lacunae. This
strain gradient may influence sequential activation of osteoclasts
and osteoblasts, with osteoclasts activated by reduced strain, and
osteoblasts by increased strain.
Bone formation typically takes 4–6 months to complete.
Osteoblasts synthesize new collagenous organic matrix, and regulate mineralization of the matrix by releasing small membranebound matrix vesicles that concentrate calcium and phosphate and
enzymatically destroy mineralization inhibitors such as pyrophosphate or proteoglycans via alkaline phosphatase [20]. Osteoblasts
surrounded by and buried within matrix become osteocytes, with
an extensive canalicular network connecting them to bone surface lining cells, osteoblasts, and other osteocytes, maintained
by gap junctions between the cytoplasmic processes extending
from the osteocytes [21]. The osteocyte network within bone
functions as a syncytium. At the completion of bone formation,
about 50–70% of osteoblasts undergo apoptosis, while remaining
osteoblasts become osteocytes or bone lining cells. Bone lining
cells may regulate influx and efflux of mineral ions into and out
of bone extracellular fluid, thereby providing a blood-bone barrier,
but also retain the ability to redifferentiate into osteoblasts upon
exposure to PTH or mechanical forces [22]. Bone lining cells within
the endosteum lift off the surface of bone prior to bone resorption
to form discrete bone remodeling compartments with a specialized
microenvironment [23].
The end result of each bone remodeling cycle is a new osteon.
Remodeling is essentially the same in cortical and trabecular bone,
with bone remodeling units in trabecular bone equivalent to cortical bone remodeling units divided in half longitudinally [24].
Bone balance is the difference between the old bone resorbed and
new bone formed. During adult life, periosteal bone balance is
mildly positive, whereas endosteal and trabecular bone balance are
slightly more negative. Endosteal resorption normally occurs faster
than periosteal formation during aging.
Bone remodeling is critical for maintenance and restoration of
bone mechanical strength by replacing older microdamaged bone
with newer healthy bone, and maintenance of calcium and phosphate homeostasis. The relatively low adult cortical bone turnover
rate of 2–3%/year is adequate to maintain biomechanical strength
of bone. The rate of trabecular bone turnover is higher than required
for maintenance of mechanical strength, indicating that it plays a
more significant role in mineral metabolism. Bone remodeling units
appear to be mostly randomly distributed throughout the skeleton,
but may be focused in areas of microcrack formation or osteocyte
apoptosis. The bone remodeling space is the sum of all the active
bone remodeling units in the skeleton at a given time.
2.2. Osteoclasts
Osteoclasts are the only recognized cells capable of resorbing bone. Activated multinucleated osteoclasts are derived from
mononuclear precursor cells of the monocyte–macrophage lineage
247
in bone marrow [25]. Bone marrow monocyte–macrophage precursor cells give rise to osteoclasts, and arrive in the bone remodeling
unit compartment via the circulation.
RANKL and M-CSF are critical for osteoclast formation. Both
RANKL and M-CSF are produced mainly by bone marrow stromal cells and osteoblasts in membrane-bound and soluble forms,
and osteoclastogenesis requires the presence of stromal cells
and osteoblasts in bone marrow [26]. RANKL belongs to the
Tumor Necrosis Factor (TNF) superfamily. M-CSF is required
for the proliferation, survival, and differentiation of osteoclast
precursors, as well as osteoclast survival and cytoskeletal rearrangement required for bone resorption. Osteoprotegerin (OPG) is
an osteoblast membrane-bound and secreted protein that binds
RANKL with high affinity to interfere with its interaction with the
RANK receptor [27].
Bone resorption depends on osteoclast secretion of hydrogen
ions and cathepsin K and other digestive enzymes across the ruffled border facing the resorption pit. While H+ ions acidify the
resorption compartment beneath osteoclasts to dissolve the mineral component of bone matrix, cathepsin K and other enzymes
digest the proteinaceous matrix, which is largely composed of type
I collagen [25].
Osteoclasts bind to bone matrix via osteoclast membranebound integrin receptors that link to bone matrix peptides. The
␤1 family of integrin receptors in osteoclasts binds to collagen,
fibronectin, and laminin in the matrix, but the main integrin receptor facilitating bone resorption is the ␣v␤3 integrin, which binds
to osteopontin and bone sialoprotein [28].
Binding of osteoclasts to bone matrix causes them to become
polarized. The bone resorbing surface develops a ruffled border
when acidified vesicles containing matrix metalloproteinases and
cathepsin K fuse to the membrane after intracellular transport to
the membrane by microtubules. The ruffled border secretes H+ ions
via H+ -ATPase and chloride channels, and releases cathepsin K and
other enzymes in the acidified vesicles [29].
After osteoclasts contact with bone matrix, the fibrillar actin
cytoskeleton of the osteoclast organizes into an actin ring, promoting formation of the sealing zone around the periphery of the
osteoclast attachment to matrix. The sealing zone surrounds and
isolates the acidified bone resorption compartment from the surrounding bone surface [30]. Actively resorbing osteoclasts form
podosomes that attach to bone matrix, with podosomes composed
of an actin core surrounded by ␣v␤3 integrins and associated
cytoskeletal proteins.
2.3. Osteoblasts
Osteoprogenitor cells give rise to and maintain the osteoblasts
that form new bone matrix on bone-forming surfaces, osteocytes
within bone matrix that sense bone biomechanical forces and
support bone structure, and lining cells that cover the surface of
quiescent bone. Self-renewing, pluripotent stem cells give rise to
osteoprogenitor cells in various tissues under the right environmental conditions. Bone marrow contains a small population of
mesenchymal stem cells capable of giving rise to bone, cartilage,
fat, or fibrous connective tissue, distinct from the hematopoietic
stem cell population that gives rise to blood cells [31]. Multipotential myogenic cells have been identified that are capable of
differentiating into bone, muscle, or adipocytes. Mesenchymal cells
committed to one phenotype may dedifferentiate during proliferation and develop into another phenotype, depending on the
local tissue environment. Blood vessel pericytes may develop an
osteoblastic phenotype during dedifferentiation under the right
circumstances [32].
Commitment of mesenchymal stem cells to the osteoblast lineage requires activity of the canonical Wnt/␤-catenin pathway
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and associated proteins [33]. Identification of kindreds with a high
bone mass phenotype associated with activating mutations of LDL
receptor-related protein 5 (LRP-5) highlights the importance of
the canonical Wnt/␤-catenin pathway in embryonic skeletal patterning, fetal skeletal development, and adult skeletal remodeling
[34,35]. The Wnt system is also important in chondrogenesis and
hematopoiesis, and may be stimulatory or inhibitory at different
stages of osteoblast differentiation.
Flattened bone lining cells are believed to be quiescent
osteoblasts forming the endosteum on trabecular and endosteal
surfaces, and underlying the periosteum on the mineralized surface. Osteoblasts and lining cells are found in close proximity,
and joined by adherens gap junctions. Cadherins are calciumdependent transmembrane proteins that are integral components
of adherens junctions, and together with tight junctions and
desmosomes, join cells together by linking their cytoskeletons [36].
Osteoblast precursors change shape from spindle-shaped
osteoprogenitors to large cuboidal differentiated osteoblasts on
bone matrix surfaces after pre-osteoblasts stop proliferating.
Preosteoblasts found near functioning osteoblasts in the bone
remodeling unit usually express alkaline phosphatase. Active
mature osteoblasts synthesizing bone matrix have large nuclei,
enlarged Golgi structures, and extensive endoplasmic reticulum.
Active osteoblasts secrete type I collagen and other matrix proteins
vectorially toward the bone formation surface.
Populations of osteoblasts are heterogeneous, with different
osteoblasts expressing slightly different sets of genes that may
explain the heterogeneity of trabecular microarchitecture seen at
different skeletal sites. Variation in gene expression also likely
explains anatomic site-specific differences in multiple disease
states, and regional variation in the ability of osteoblasts to respond
to agents used to treat bone disease.
2.4. Bone loss
As a result of the age-related changes in osteoclast and
osteoblast function described, bone density and structure deteriorate over time and lead to increased fracture risk in both women and
men. Distal forearm (Colles’) fractures increase rapidly in women
after menopause, and then remain relatively constant from about
10 to 15 years after menopause until the end of life. In contrast, vertebral fractures increase more slowly after menopause,
but continue to increase exponentially during later life. Hip fractures in women increase more slowly than vertebral fractures
after menopause, but continue to increase throughout life also,
and increase rapidly in later life. In men, however, distal forearm
fractures do not appear to increase with normal aging, probably
because larger bone size protects against them. Vertebral and hip
fractures increase gradually with aging in men, beginning about
a decade later compared to women, likely due to their relative
preservation of gonadal steroid production until later in life.
It is estimated that 40% of Caucasian women aged 50 years or
older will develop a vertebral, hip, or wrist fracture sometime during the remainder of their lives, and that this risk increases to about
50% if vertebral fractures detected only by radiological imaging are
included in the estimate [37]. It is estimated that about 13% of Caucasian men will sustain similar fractures. The risk of these fractures
is somewhat lower in non-Caucasian women and men. It is estimated that osteoporotic fractures cost the U.S. between $12.2 to
$17.9 billion each year, as measured in 2002 dollars [38].
3. Signaling pathways
Signaling pathways are critical in regulation of how osteoclasts and osteoblasts control bone turnover leading to bone loss
after menopause and during normal aging. Activation of pathways that stimulate osteoclast recruitment and activation lead to
bone loss unless other factors prevent this, and those that stimulate osteoblast recruitment and activation lead to bone formation,
unless other factors prevent this. Whereas currently available
agents used to treat osteoporosis mostly inhibit bone resorption,
newer agents will largely focus on targets in signaling pathways
that stimulate osteoblast function.
4. Antiresorptive pathways
Antiresorptive agents reduce bone turnover relatively rapidly,
by reducing bone resorption initially, follow by a slower reduction in bone formation. These changes lead to an altered balance
between bone resorption and formation at a lower rate of bone
turnover that favors an increase in BMD, preservation or strengthening of structural and material properties of bone, increase in bone
strength, and reduction in fractures [39].
4.1. Bisphosphonates and denosumab
Nitrogen-containing bisphosphonates block osteoclast function
by inhibiting farnesyl pyrophosphate synthase, an enzyme in the
mevalonate cholesterol synthesis pathway which is critical for
membrane protein prenylation, thereby disrupting the osteoclast
cytoskeleton and leading to osteoclast detachment from bone,
reduced bone resorption, and osteoclast apoptosis [40]. Denosumab inhibits RANKL produced by osteoblasts from stimulating
osteoclast activation, thereby leading to reduced differentiation
and activation of osteoclasts, as well as apoptosis [41].
4.2. Cathepsin K
Recognition of signaling pathway targets has led to the development of the newer antiresorptive agents. Cathepsin K is a cysteine
protease that cleaves the triple helix of type 1 collagen, in addition to cleaving collagen telopeptides from the N- and C-termini
of this collagen. These actions contribute to resorption of the proteinaceous bone matrix containing type 1 collagen [42]. Cathepsin
K inhibitors have been developed to block the action of this enzyme
and limit bone resorption.
5. Anabolic pathways
Anabolic agents stimulate bone turnover significantly and relatively rapidly by stimulating bone formation initially, followed by
slower stimulation of bone resorption. These changes lead to an
altered balance between bone formation and resorption, at a higher
rate of bone turnover, which leads to an increase in BMD, preservation or strengthening of structural and material properties of bone,
increase in bone strength, and reduction in fractures.
5.1. PTH
Teriparatide (rhPTH 1–34) is the only anabolic agent approved
for treatment of osteoporosis in the U.S. Recombinant human PTH
1–84 is approved for treatment of osteoporosis in Europe and other
countries. Both of these PTH peptides stimulate osteoblast function by binding to the PTH/PTHrP type 1 receptor and activating
several signaling pathways, including the canonical Wnt-signaling
pathway. These pathways are stimulated, in part, by PTH actions
expressed through PTH/PTHrP type 1 receptors on osteocytes
[43].
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249
5.2. PTHrP
5.5. Bone morphogenetic proteins
PTHrP peptides such as PTHrP 1–36 or PTHrP 1–34
(abaloparatide) bind to the PTH/PTHrp type 1 receptor also,
and activate signal transduction with equal potency to PTH. In a
clinical trial, PTHrP 1–36 appeared to stimulate bone formation
with equal potency to PTH, but led to reduced increases in bone
resorption [44].
Bone morphogenetic proteins (BMPs) stimulate bone formation
when given locally by injection for non-union of fractures. Unfortunately, BMP signaling occurs in many tissues, is mitogenic, and
has a short half-life, making it less useful for treatment of systemic
osteoporosis. Proteosome inhibitors are chemotherapeutic agents
that stimulate BMP-2 expression, so these may have potential benefits in bone tissue, and could serve as anabolic therapies. Although
growth hormone and insulin-like growth factor-1 also stimulate
bone formation, these hormones also act in many other tissues,
limiting their applicability for treatment of osteoporosis.
5.3. Wnt signaling pathway
Stimulators of the Wnt-signaling pathway are being actively
explored for their anabolic effects on bone [45] (Fig. 1). Sclerostin
is produced by osteocytes during late stages in their differentiation, after the start of mineralization of the surrounding matrix
[46]. Newly synthesized sclerostin is transported to the bone surface by the canalicular network of osteocytes within bone, and
inhibits osteoblast function and stimulates osteoblast apoptosis
[47,48]. Sclerostin also plays an autocrine role and upregulates
RANKL synthesis in osteocytes, leading to stimulation of osteoclastogenesis [49]. Sclerostin antagonizes the Wnt-signaling pathway
in osteoblasts leading to reduced osteoblast proliferation, differentiation, and survival [50–52].
Wnt ligands normally bind to LRP5/6 and Frizzled co-receptors
at the cell surface to cause transduction of a signal that stimulates
activation and accumulation of intracellular beta-catenin, which
translocates to the nucleus and stimulates transcription of target
genes in osteoblasts (Fig. 2). Sclerostin binds to the first propeller
domain of the LRP5/6 receptor, inhibiting the formation of the
LRP5/6–Frizzled co-receptor complex, which results in inhibition
of the Wnt-signaling pathway [51]. Sclerostin may require LRP4 as
a cofactor to be fully functional, similar to Kremen, which is necessary for another Wnt pathway inhibitor, Dkk1, to fully inhibit the
Wnt signaling pathway. Biomechanical strain and high PTH levels
cause downregulation of sclerostin production in osteocytes, and
lead to stimulation of bone formation [53].
Dickkopf 1 (Dkk1) also binds to the extracellular domains of
LRP5/6 to prevent interaction with Wnts, which leads to inhibition of the formation of the LRP5/6–Frizzled co-receptor complex,
resulting in inhibition of the Wnt-signaling pathway [54]. Dkk1
is produced by osteocytes, but also by other cells in the body,
and may not be expressed in the bones of older animals. These
observations may make Dkk1 inhibitors potentially less useful as
treatments for osteoporosis. Because Dkk1 is secreted by multiple
myeloma cells to block osteoblasts from filling in resorbed bone in
lytic lesions, Dkk1 inhibitors are being explored as treatments for
multiple myeloma.
Other more distal targets within the Wnt-signaling pathway
may also be useful in treatment of osteoporosis. Glycogen synthase
kinase (GSK)-3␤ is central in the Wnt-signaling pathway, with
inhibition of GSK-3␤ stabilizing beta-catenin and activating later
steps in the pathway. Lithium chloride inhibits GSK-3␤, resulting
in increased bone formation and bone mass in mice [55]. GSK-3␤ is
also involved in other signaling pathways, however, and numerous
growth factors regulate its activity, limiting its utility as a target for
treatment of osteoporosis.
5.4. Activin A
Activin A is a member of the TGF␤ superfamily that has been
reported to inhibit bone mineralization and stimulate osteoclastogenesis [56]. Its mechanism of action is not yet fully understood.
Soluble activin A type II receptor (ActRIIA) has been shown to block
this signaling pathway and to prevent inhibition of bone formation.
5.6. Nitroglycerin
Nitroglycerin, which stimulates the nitric oxide pathway, has
been shown to simultaneously stimulate bone formation and
inhibit bone resorption. Epidemiologic studies have shown that
patients using nitrate therapy have increased BMD compared to
those not using it. Women treated with nitroglycerin ointment for
two years had modestly increased bone mineral density, with a
decrease in urinary NTX and non-significant increase in serum bone
alkaline phosphatase [57], but the study was not powered to detect
fractures.
6. Therapeutic targets for osteoporosis
Newer therapies for osteoporosis are targeted to molecules discovered during elucidation of signaling pathways in osteoclasts and
osteoblasts (Table 1). The limitations of new therapies are mostly
related to their specificity of action and safety. Agents that also
affect tissues other than bone may result in off-target effects, which
cause adverse events in clinical trials.
7. Antiresorptive agents
Most currently approved therapeutic agents primarily prevent
bone loss. There is continued interest in development of new antiresorptive agents that offer selective advantages over existing agents.
Because newer antiresorptive agents do not markedly suppress
bone formation, and may uncouple bone formation from resorption, these compounds offer the potential of creating anabolic
windows of variable magnitude and duration to stimulate new
bone formation.
7.1. Cathepsin K inhibitors
The category of new antiresorptive agent farthest along in development is cathepsin K inhibitors [58,59]. Cathepsin K is a major
digestive enzyme that breaks down type I collagen secreted by
activated osteoclasts during bone resorption. Because cathepsin
K inhibitors selectively target one of the main osteoclast digestive enzymes, but do not inhibit other digestive enzymes, or block
acid secretion by actively resorbing osteoclasts, their antiresorptive
effect is milder than that of more potent antiresorptive agents such
as bisphosphonates or denosumab that markedly decrease both
bone resorption and bone formation. Cathepsin K inhibitors, like
most of the newer agents, have rapid offset of action, so patients
taken off this type of drug will require addition of another medication to prevent loss of bone previously gained with cathepsin K
inhibitor treatment.
Odanacatib is a new selective cathepsin K inhibitor that causes
a moderate sustained decrease in bone resorption, but a lesser
and more transient decrease in bone formation in completed clinical trials in postmenopausal women [60–62]. Preclinical studies
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Fig. 1. Wnt signaling: a simplified view. In the absence of Wnt, the amounts of ␤-catenin are low, except in adherens junctions, because of its constitutive targeting
by a multiprotein destructive complex (left). The tumor suppressors axin and adenomatous polyposis (APC) bring ␤-catenin to GSK-3␤ and casein kinase 1 (CK1) (not
shown), resulting in its phosphorylation (P’s in circles) at specific serine/threonine residues (left). Phosphorylated ␤-catenin is then targeted for polyubiquitination (Ub)
(predominantly by the E3 ligase B-TrCP) and proteosomal destruction. T cell factor/lymphoid enhancer factor (TCF/LEF) transcription factors are repressed by Groucho in the
nucleus. Binding of canonical WNT ligands to a dual receptor complex comprising the co-receptors LRP5 or LRP6 (LRP5/6) and one of the seven transmembrane receptors of
the FZD family (right) initiates WNT-␤-catenin signaling. Axin moves to the LRP 5/6 tail at the membrane through its interaction with disshevelled (DVL, also called DSH),
which is recruited by FZD (right). This forms a complex that also includes FRAT-1 and GSK-3␤, which prevents phosphorylation of ␤-catenin and its proteosomal degradation.
␤-catenin accumulates in the cytoplasm and translocates into the nucleus, where it associates with members of the TCF/LEF transcription factors while displacing Groucho
to control target gene expression. WNT signaling is regulated not only through tuning by a large number of WNT ligands and RSPO proteins and norrin (right) but also by
extracellular antagonists such as DKK1, SOST and Wise, which bind to LRP5/6 (left). Their antagonism is mediated or enhanced by receptors such as the Kremen protein and
LRP4. In addition to sequestering ␤-catenin, N-cadherin also inhibits WNT-␤-catenin signaling by interacting with LRP5 (left to right). In contrast, secreted frizzled related
proteins (SFRPs) and WNT inhibitory factor 1 (WIF 1), which have ligand specificity, inhibit WNT signaling by directly sequestering WNT ligands and inhibiting both canonical
and non-canonical WNT signaling (left). The PTH1 receptor can also activate the pathway in the absence of WNT ligands by forming a complex with LRP5/6 after PTH binding
(right). In the WNT-PCP pathway (right), WNT binding to FZD also recruits DVL, which forms a complex with disshevelled associated activator of morphogenesis (DAAM1)
to trigger activation of the small G protein RHO, which in turn activates RHO-associated kinase (ROCK). Alternatively, DVL forms a complex with RAC, resulting in Jun kinase
(JNK) activity. The WNT-Ca2+ pathway is activated by WNT5a binding to FZD and receptor-tyrosine-kinase-like orphan receptor (ROR). Intracellular calcium concentrations
increase after WNT-induced coupled G protein activation of phospholipase C (PLC), resulting in diacylglycerol (DAG) and inositol 1,4,5 trisphosphate, type 3 (IP3) generation
and cyclic GMP (cGMP)-specific phosphodiesterase (PDE) decreasing the amount of cGMP. NFATc1, nuclear factor of activated T cells, cytoplasmic, calcineurin dependent 1;
PPAR-␥, peroxisome proliferator activated receptor-␥.
Source: Reproduced by permission from Baron and Kneissel [45].
in mice, rabbits, or monkeys showed that cathepsin K deficiency
led to maintenance of, or an increase, in bone formation [63,64].
These changes resulted in a moderate increase in BMD, similar to
what is seen with alendronate. Odanacatib has completed phase III
clinical trials in postmenopausal women and older men in the U.S.,
and is undergoing adjudication of adverse events requested by the
FDA. Odanacatib does not decrease osteoclast survival or limit bone
formation, unlike bisphosphonates and other available antiresorptive agents, and appears to uncouple bone resorption from bone
formation. This uncoupling leads to maintenance of bone formation with moderately decreased bone resorption, resulting in an
anabolic effect.
ONO-5334 is another cathepsin K inhibitor that has completed
phase I and II clinical trials [65,66]. The phase II OCEAN clinical
trial [66] demonstrated that this agent decreased bone resorption
similar to alendronate, with little or no change in bone formation,
in a dose ranging study. Lumbar spine, femoral neck, and total hip
BMD increased comparably to what was seen with alendronate.
No clinically relevant safety concerns were identified in the
trial.
Early cathepsin K inhibitors demonstrated off-target effects
including plaque-like skin thickening (morphea) that led to discontinuation of their clinical development. Balacatib was reported
to cause morphea [67], perhaps because of cathepsin K inhibition not just in osteoclasts, but also in skin and pulmonary
fibroblasts. Similar effects have not been seen to date with
odanacatib, ONO-5334, or other cathepsin K inhibitors still under
development.
N.M. Iñiguez-Ariza, B.L. Clarke / Maturitas 82 (2015) 245–255
251
Fig. 2. WNT signaling pathway members regulate bone mass: lessons from mouse genetics. Across all studies, increases in bone mass were observed as a result of pathway
activation, and decreases in bone mass were observed as a result of pathway inhibition, although the relative impact on bone formation and resorption varied, reflecting the
complex fine tuning of the WNT regulatory network. Alteration in the gene dosage of members of the pathway altered bone formation and resorption to varying degrees.
Obl, osteoblast; Ocy, osteocyte; Obl/Ocy, osteoblast and osteocyte; TG, transgenic; gof, gain-of-function; OVX, ovariectomy; ECD, extracellular domain; , deletion; BAC,
bacterial artificial chromosome. Blue and red boxes indicate an effect on formation and resorption, respectively, and purple boxes indicate an effect on both. The direction
of the arrows in the text boxes indicates an increase or decrease of bone resorption (red) or bone formation (blue). (For interpretation of the references to color in this figure
legend, the reader is referred to the web version of this article.)
Source: Reproduced by permission from Baron and Kneissel [45].
8. Anabolic agents
Anabolic agents increase bone strength by directly stimulating
new bone formation. A number of potential targets for anabolic
agents have been identified [68]. New parathyroid hormone (PTH)
and PTH-related protein (PTHrP) analogues remain under development. New anabolic targets being evaluated stimulate the
Wnt-signaling pathway in osteoblasts. Monoclonal antibodies to
these targets block inhibition of the pathway, leading to stimulation
of new bone formation.
8.1. PTH analogues
The only anabolic therapies currently approved to treat osteoporosis are recombinant forms of human PTH. Human recombinant
PTH 1–34 (teriparatide) [69] and PTH 1–84 [68] have been approved
in many countries to treat osteoporosis. Stopping treatment with
these agents leads to rapid bone loss within six months, so patients
are usually switched to long-acting bisphosphonates or other
antiresorptives agents to consolidate BMD gained during treatment
[70].
PTH analogues are given by daily subcutaneous injection. Their
most common side effects are mild asymptomatic hypercalcemia
and hypercalciuria [70,71]. Teriparatide was given a black box
warning by the FDA because of an increased risk of osteogenic sarcoma in the Fischer 344 rat in one preclinical study. There have
been no human osteogenic sarcoma cases to date known to be
caused by teriparatide or other PTH analogues [72]. PTH analogues
are expensive to produce, and cost more than other agents used to
treat osteoporosis.
A relatively recent clinical trial in Japan showed that higherdose teriparatide given by once weekly subcutaneous injection
significantly increased BMD [73]. Once monthly and once yearly
administration of PTH fused to a collagen-binding domain fusion
protein has been shown to lengthen its anabolic activity in mice
[74,75]. The pharmacokinetic profile of a single dose of a novel oral
PTH formulation has been evaluated in healthy postmenopausal
women [76]. Tablet and transdermal patch formulations are being
developed.
Because intermittent subcutaneous administration of PTH
causes anabolic effects on bone, short-term intermittent antagonism of the parathyroid calcium-sensing receptor (CaSR) by
calcilytic compounds may result in short bursts of endogenous PTH
secretion. Ronacaleret, an oral CaSR antagonist, transiently stimulated PTH secretion, but PTH release was prolonged enough to
cause bone loss, similar to what is seen in primary hyperparathyroidism [77]. A preclinical study showed that JTT-305, another oral
CaSR antagonist, stimulated transient PTH secretion and new bone
formation in ovariectomized rats [78].
8.2. PTH-related protein analogues
PTHrP was initially identified to be the main cause of hypercalcemia of malignancy. Intermittent injection of recombinant
analogues of PTHrP is being investigated to see if these might
improve BMD and reduce fractures. In a previous clinical trial,
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Table 1
Newer biologic agents being developed for treatment of osteoporosis.
Newer biologic agents
Agent
Mechanism of action
Dose and frequency
Treatment effect
Denosumab
Anti-RANKL monoclonal
antibody, with ↓bone
resorption and ↑BMD
60 mg SC q 6 months
- FREEDOM trial showed ↓risk of new
radiographic vertebral fractures by 68%,
hip fractures by 40%, and nonvertebral
fractures by 20%
- 8 years of continuous treatment: LS BMD
↑16.5%, and TH ↑6.8 % Reversible after
stopping treatment
Cathepsin K inhibitors:
Odanacatib
ONO-5334
Inhibit bone matrix
dissolution, ↓bone resorption
Odanacatib 50 mg po q week
ONO-5334 300 mg po q day
maximum dose
- ↓bone resorption by 50%, ↓bone
formation by 30%
- In phase 2 study, Odanacatib ↑LS BMD
by 11.9%, TH ↑8.5%, and FN ↑9.8%
- Fractures ↓
- Adverse events being adjudicated
- Reversible after stopping treatment
Anabolics
Teriparatide
(rhPTH (1–34))
↑Bone formation more than
↑bone resorption
20 mcg SC q day
- ↑Bone formation and ↑ bone resorption
- In phase 3 study, 20 mcg dose ↑LS BMD
by 9%, and ↑FN by 3% by 21 months
- ↓Vertebral and nonvertebral fractures.
- Offset of action occurs rapidly
- Use limited to 2 years in U.S., 18 months
in Europe
PTHrp Analogue (rhPTHrP
(1–34); abaloparatide)
↑Bone formation and ↑bone
resorption, ↑BMD
80 mcg SC q day maximum
dose
- In phase 2 study, 80 mcg dose ↑LS BMD
by 6.7%, ↑FN by 3.1%, and TH by 2.6% at
24 weeks
- Offset of action occurs rapidly
- Phase 3 fracture efficacy trial completed
Anti-sclerostin antibodies:
Romosozumab
Blosozumab
Sclerostin inhibition
↑Bone formation on quiescent
surfaces, ↓bone resorption
Romosozumab 210 mg SC q
month maximum dose
Blosozumab 270 mg SC q 2
weeks maximum dose
- In phase 2 study, romosozumab ↑LS
BMD by 11.3%
- Transitory ↑bone-formation markers
and sustained ↓bone resorption markers
- Offset of action occurs rapidly
- Phase 3 fracture efficacy trials ongoing
Anti-DKK1 antibodies: BHQ880
Dkk1 inhibition
BHQ880 10 mg/kg dose for
phase 2 trial
- Phase 1 trial completed for treatment of
lytic bone lesions in multiple myeloma
- Dkk1 production by multiple myeloma
cells ↓bone formation in lytic bone
lesions
- Dkk1 expression not restricted to bone
Abbreviations: LS, lumbar spine; FN, femoral neck; TH, total hip; BMD, bone mineral density.
intravenous rhPTHrP 1–36 was shown to stimulate bone formation
effectively, with a reduced increase in bone resorption, compared to
rhPTH 1–34 [44]. Human recombinant PTHrP 1–34 (abaloparatide)
was previously shown to have similar effects to teriparatide in a
small clinical trial. In a larger clinical trial, the same agent was
recently shown to cause significant increases in lumbar spine,
femoral neck, and total hip BMD in a dose-dependent fashion with
daily subcutaneous injection for 24 weeks [79]. The increase seen
at the total hip was greater than seen with the teriparatide control.
8.3. Activators of the Wnt/ˇ-catenin signaling pathway
Binding of Wnt to its 7-transmembrane receptor and LRP 5/6
in osteoblasts causes inhibition of formation of intracellular GSK3␤. Inhibition of Wnt binding leads to increased GSK-3␤, which
leads to breakdown of ␤-catenin. Decreased GSK-3␤ activity leads
to increased translocation of ␤-catenin to the nucleus, causing transcriptional co-activation of genes integral to bone formation [80].
An anti-sclerostin monoclonal antibody (AMG 785) stimulated
bone formation and increased BMD and bone strength in a rat preclinical study [81]. A randomized, placebo-controlled phase I study
in 72 healthy adults showed that a single subcutaneous dose of
anti-sclerostin antibody (romosozumab) increased BMD [82]. Subcutaneous doses ranging from 0.1 to 10 mg/kg were studied in 56
subjects, with intravenous doses of 1 or 5 mg/kg studied in 16 subjects. A single dose of romosozumab by either the subcutaneous
or intravenous route caused a dose-dependent increase in markers of bone formation, and a dose-dependent decrease in a marker
of bone resorption. A dose-dependent increase in BMD was seen as
early as one month after administration. The largest BMD increases
seen were 5.3% at the lumbar spine, and 2.8% at the total hip, on
day 85 after a single 10 mg/kg subcutaneous dose. Romosozumab
was well tolerated at all doses. Adverse events were mild, except
for one subject who received the 10 mg/kg dose, who developed
severe non-specific hepatitis. Liver enzymes were increased by the
first day after dosing, but normalized by day 26.
Efficacy and safety of romosozumab was evaluated in a phase
2 randomized, placebo-controlled, parallel-group study over 12
months in 419 postmenopausal women who had low BMD
[83]. Participants were randomly assigned to receive subcutaneous romosozumab monthly or every 3 months, subcutaneous
placebo, open-label active comparator with oral alendronate
N.M. Iñiguez-Ariza, B.L. Clarke / Maturitas 82 (2015) 245–255
(70 mg weekly), or subcutaneous teriparatide (20 mcg daily). All
dose levels of romosozumab caused significant increases in BMD
at the lumbar spine, including an increase of 11.3% with the 210mg monthly dose, as compared with a decrease of 0.1% with
placebo, and increases of 4.1% with alendronate and 7.1% with teriparatide. Romosozumab was also associated with large increases
in bone mineral density at the total hip and femoral neck, as well
as transitory increases in bone-formation markers and sustained
decreases in a bone-resorption marker. Except for mild, generally
non-recurring injection site reactions with romosozumab, adverse
events were similar among groups.
Blosozumab, another anti-sclerostin antibody, was evaluated
in a randomized, double-blind, placebo-controlled phase 2 clinical
trial in postmenopausal women with low BMD [84]. The study randomized 120 subjects to subcutaneous blosozumab 180 mg every 4
weeks, 180 mg every 2 weeks, 270 mg every 2 weeks, or matching
placebo for 1 year, with calcium and vitamin D supplementation.
Blosozumab treatment resulted in statistically significant doserelated increases in spine, femoral neck, and total hip BMD as
compared with placebo. In the highest dose group, BMD increases
from baseline reached 17.7% at the spine, and 6.2% at the total hip.
Biochemical markers of bone formation increased rapidly during
blosozumab treatment, and trended toward pretreatment levels by
the end of the study. However, bone specific alkaline phosphatase
remained higher than placebo at study end in the highest-dose
group. CTx, a biochemical marker of bone resorption, decreased
early in blosozumab treatment to a concentration less than that of
the placebo group by 2 weeks, and remained reduced throughout
blosozumab treatment. Mild injection site reactions were reported
more frequently with blosozumab than placebo.
To evaluate the effect of discontinuing blosozumab, women
enrolled in the 1-year randomized, placebo-controlled phase 2 trial
were followed for an additional year [85]. At the end of follow-up,
lumbar spine and total hip BMD decreased, but remained significantly greater than placebo in women initially treated with
blosozumab 270 mg every 2 weeks, and blosozumab 180 mg every
2 weeks. During follow-up, median serum P1NP was not consistently different between the prior blosozumab groups and placebo.
A similar pattern was apparent for median serum CTx levels,
with more variability. Mean serum total sclerostin concentration
increased with blosozumab, indicating target engagement, and
declined to baseline after discontinuation. Anti-drug antibodies
generally declined in patients who had detectable levels during
prior treatment.
Monoclonal antibodies to Dickkopf-1 are being developed. One
of these, RH2-18, was given subcutaneously to ovariectomized mice
and rhesus macaques, and caused increased BMD, especially at trabecular sites [86]. Eight weeks of weekly injections of RH2-18 in
ovariectomized mice increased femoral BMD to the level of controls
without ovariectomy, and partially restored lumbar spine BMD
toward that of controls. RH2-18 given to mice every two weeks
for 9 months increased lumbar spine BMD by 5.0%.
253
[88]. There have been no randomized controlled trials in humans
evaluating the use of these agents for treatment of osteoporosis.
9. Conclusions
Major advances have occurred recently in the treatment of
osteoporosis. Patients are most often treated currently with bisphosphonates, denosumab, raloxifene, or teriparatide, and in some
countries, strontium ranelate. Calcitonin has been removed from
the market in some countries, and its use restricted in others, due
to risk of malignancy. Use of strontium ranelate has been reduced
due to cardiovascular concerns. These agents have generally provided excellent options that effectively reduce fracture risk. The
signaling pathways described in this review, with the new agents
targeting these signaling pathways, will further expand the options
available for treatment of osteoporosis.
Practice points
• Until the newer anabolic or antiresorptive agents are approved,
physicians should continue to prescribe the current best agent
uniquely suited for each patient.
• Use of the currently available agents should not preclude the use
of new agents when these become available.
• Physicians and patients should be aware that, unlike the longeracting bisphosphonates, all the newer agents are relatively
shorter-acting, and have reasonably rapid offset of action.
• Physicians should consider using longer-acting bisphosphonates
or denosumab once treatment with the new shorter-acting
agents is completed.
Research agenda
• Continue clinical trials of the available new antiresorptive and
anabolic agents.
• Continue development of newer therapeutic agents currently
undergoing preclinical evaluation.
• Continue preclinical research on agents targeting anabolic pathways other than the Wnt/␤-catenin signaling pathway to expand
options for treatment.
Conflict of interest
The authors declare no conflict of interest.
Authors’ contribution
Both authors contributed equally to the design, writing, and
editing of this narrative review, submitted at the invitation of the
Editor-in-Chief, Dr. Margaret Rees.
Funding
8.4. Other anabolic agents
A variety of new targets are currently being evaluated in preclinical studies. Potential targets include growth factors, such as
bone morphogenetic proteins (BMPs) and transforming growth
factor-␤ (TGF-␤). Recombinant human BMP-2 has been given
locally by injection to patients with bisphosphonate-associated
jaw osteonecrosis, and found to stimulate bone formation sufficiently to heal these difficult to treat lesions [87]. Statins and
statin-like molecules have been shown to stimulate BMP-2 gene
expression and vascular endothelial growth factor (VEGF) expression in osteoblasts, and to stimulate fracture healing in animals
The authors have received no funding for this article.
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