Lasers in Medical Science 1996, 11:261-265
Light and Drug Distribution with Topically
Administered Photosensitizers
L.O. SVAASAND a'b, B.J. TROMBERG b, P. WYSS b'c, M.-T. WYSS-DESSERICH b'c, Y. TADIR b,
M.W. BERNS b
aNorwegian University of Science and Technology, Trondheim, Norway
bBeckman Laser Institute, University of California, Irvine, USA
cUniversity of Z(Jrich, Switzerland
Correspondence to L.O. Svaasand, Division of Physical Electronics, Norwegian University of Science and Technology,
Trondheim, N-7034, Norway
Paper received 30 June 1996
A b s t r a c t . Photodynamic therapy (PDT) based on topical application of photosensitizers is
currently in clinical use for the treatment of basal cell carcinoma of the skin, and it has been
evaluated in animal models for photo-ablation of the endometrium. This paper presents a dosimetry
model which indicates that a limiting factor in treating thick tumours will be the transport of the
drug into the tumour rather t h a n depletion of the optical distribution. The model predicts that an
optical irradiation of 100mW cm -2 at 635 nm for 20min, ie well below the threshold for
hyperthermic reaction, will give an adequate light dose to a depth of 3 mm. The time required for
photosensitizers to diffuse to this depth is in the range of 3-15 h, dependent on the diffusion
properties of the tissue.
INTRODUCTION
Effective
photodynamic
therapy
(PDT)
requires the generation of cytotoxie products
by excited-state photosensitizers (1-3). Most
photosensitizers, eg haematoporphyrin derivatives, chlorins and phthalocyanines, have
traditionally worked best when administered
systemically. In recent years, however, very
promising results have been obtained with
topical application of 5-aminolaevulinic acid
(5-ALA). This compound, a precursor in the
biosynthesis of haem, stimulates the production of the photodynamically active compound,
protoporphyrin IX (PpIX). Topically administered sensitizers have been used clinically in
photodynamic treatment of basal cell carcinomas, and experimentally in animal models for
destruction of the endometrium.
In PDT with systemically administered
sensitizer, it is usually appropriate to assume a
uniformly distributed drug dose in the target
tissue (4, 5). The decay of the cytotoxic dose
with distance from the irradiated surface is
therefore primarily due to depletion of the
light.
0268-8921/96/040261 +05 $12.00/0
The situation is almost the opposite when
the sensitizer is applied topically. The distribution of the drug with distance from the surface
is dependent on molecular diffusion properties
and tissue vascularity (6). Since the penetration depth of the drug is usually smaller than
the optical penetration depth, drug transport
properties, rather t h a n tissue optical properties, can limit the treatment of thick, deep
lesions.
OPTICAL DISTRIBUTION
The optical absorption and scattering coefficients of skin are quite different from those of
other tissues. One of the characteristics of skin
is the very high scattering coefficient, ie the
effective (reduced) scattering coefficients of
dermis and epidermis are typically a factor of
5-10 larger t h a n the values found in muscle
tissue and in many turnouts. The effective
back-scattering of light in skin results in a
high reflection coefficient together with an
elevated optical fluence rate in epidermis and
in upper dermis. However, the high scattering
9 1996 W.B. Saunders Company Ltd
L.O. Svaasand, B.J. Tromberg, P. Wyss et al
262
3.5
~
2.8
3
2.6
2.5
~ 1.5
~ 2.4
~
(a)
2.2
"~....
1.8
0.5
1
1.5
2
Depth (mm)
2.5
3
0.5
0
1.8 1.6 ~
~ 1.4
~ 1.2
2.5
2
1
-,
,
1.5
2
Depth (mm)
2.5
3
2.5
3
,
"-,,,
1.5
~ 0.8
~ 0.6
0.4
1
0.5
(b)
I I I I
0
0.5
"~-~ ....
1
1.5
2
Depth (ram)
J~,,,
2.5
",
(b)
I,
3
Fig. 1. Light dose distribution in fair Caucasian skin.
Incident fluence normalized to unity. (a) Wavelength
635 nm, (b) wavelength 514 nm. Epidermal thickness
1O0/zm. Dermal blood fraction 1% ( - - - ) and 2% (
).
Optical properties at 635 nm: dermal scattering coefficient
,u~=23 9 103 m -~, epidermal scattering coefficient
/~=45 9 103 m -~, average cosine of scattering angle g=0.8,
dermal absorption coefficient (in absence of blood)
/~=25 m -~, epidermal absorption coefficient (melanin)
/z~=570 m -1. Optical properties at 514 nm: dermal
scattering coefficient,u~=28 9 103 m -~, epidermal scattering
coefficient/~=56 9 103 m -~, average cosine of scattering
angle g=O.8, dermal absorption coefficient (in absence of
blood)/~=25 m -1, epidermal absorption coefficient
(melanin)/~a=1.33 9 103 m -~.
reduces the optical penetration depth, and the
optical fluence rate at depths larger t h a n
1-2 mm in skin will be smaller t h a n in the case
of most o th er tissues. The detailed optical
distribution in skin is very complex, but
mathematical modelling based on numerical
Monte Carlo calculations or analytical models
based on diffusion theory give a fairly good
description (7, 8).
The optical distribution in fair Caucasian
skin is given in Fig. 1 (8). Figure l(a) shows
t h a t the distribution for 635 nm light peaks in
the upper dermis at a level of about four times
the incident fluence. The in situ fluence level
remains above the incident unscattered fluence to a depth of about 2.5 mm (it is assumed
here t h a t the optical properties of any tumour
0
0.5
1
1.5
2
Depth (ram)
Fig. 2. Light dose distribution in tissue covered by
epidermis. Fair Caucasian skin. Incident fluence normalized
to unity. (a) Wavelength 635 nm, (b) wavelength 514 nm.
Epidermal thickness lO0,um. Dermal blood fraction 1%
( - - - ) and 2% (
). Optical properties at 635 nm and
514 nm: reduced (effective) scattering coefficient/zs=,u8
( 1 - g ) = l . 103 m -1, Tumour absorption coefficient (in
absence of blood) ,ua=25 m -1, epidermal absorption
coefficient (melanin)/za=570 m -1 at 635 nm, and
/~a=1.33 9 103 m -1 at 514 nm.
and/or the subcutaneous fat are approximately
the same as those of the dermis). The major
absorber in epidermis is melanin, whereas the
predom i nant chromophore in dermis is haemoglobin. The corresponding results at 514nm
are shown in Fig. l(b). The enhanced absorption in both haemoglobin and melanin at
514 nm compared to 635 nm reduces the peak in
situ fluence to about three times the incident
fluence. It is also n o t e w o r t h y t hat at 514 nm,
the fluence now peaks in the stratum corneum
r a t h e r t h a n in the upper dermis. The fluence
remains above incident to about 0.5-0.7mm
depth, but falls off substantially at depths
larger t h a n 1-1.5 mm.
The corresponding optical distributions in
tissues with more typical optical properties,
such as would be expected for many tumours,
are shown in Fig. 2. The melanin content of
epidermis is kept unchanged, but the epidermal scattering coefficient is t aken to be equal
Topical Administration of Photosensitizers
to t h a t of t h e u n d e r l y i n g tissues. F i g u r e 2(a)
shows t h e r e s u l t s for 635 n m light; the r e d u c e d
b a c k - s c a t t e r i n g n o w lowers t h e m a x i m u m
v a l u e of the in situ fiuence to a b o u t t h r e e times
the incident. H o w e v e r , t h e fluence r e m a i n s
a b o v e i n c i d e n t levels e v e n a t d e p t h s l a r g e r
t h a n 3 mm.
T h e c o r r e s p o n d i n g r e s u l t s for 514 n m light
are s h o w n in Fig. 2(b). T h e fiuence in t h e
e p i d e r m i s is a b o u t a f a c t o r of t w o l a r g e r t h a n
t h e incident, a n d it r e m a i n s a b o v e t h e i n c i d e n t
v a l u e to a d e p t h of I 1.5 mm.
A t y p i c a l i n c i d e n t o p t i c a l dose for P D T w i t h
red l i g h t at 630-635 n m is a r o u n d 100 J c m - 2
This dose c a n easily be o b t a i n e d w i t h o u t
e x c e e d i n g t h e t h r e s h o l d for h y p e r t h e r m i a , eg
w i t h a n i r r a d i a n c e of 100 m W c m - 2 for 1000 s
(17 min). T h e in situ o p t i c a l dose will be a b o v e
this v a l u e for d e p t h s of up to 2.5 m m in h i g h l y
s c a t t e r i n g tissues s u c h as t h e dermis, w h e r e a s
the d e p t h c a n be s i g n i f i c a n t l y d e e p e r in s k i n
t u m o u r s w i t h less s c a t t e r i n g [see Figs l(a)
a n d 2(a)].
I r r a d i a t i o n w i t h g r e e n light at 514 n m m a y
r e p r e s e n t a v e r y i n t e r e s t i n g o p t i o n for s h a l l o w
lesions. T h e in situ fluence will be a b o v e the
i n c i d e n t o p t i c a l dose to a d e p t h of a b o u t
0.5 m m in h i g h l y s c a t t e r i n g tissues, like norm a l dermis, a n d to 1-1.5 m m in tissues w i t h
m o d e r a t e s c a t t e r i n g , like t u m o u r , p r o v i d e d
t h a t the b l o o d c o n t e n t is n o t too large.
T R A N S P O R T OF 5 - A L A
T h e r a t e of diffusion of a d r u g is d e t e r m i n e d by
the c o n c e n t r a t i o n g r a d i e n t s a n d by a c o n s t a n t
u s u a l l y r e f e r r e d to as t h e diffusion c o n s t a n t or
the diffusivity, K. T h i s c o n s t a n t is d e p e n d e n t
on the p r o p e r t i e s of t h e tissue as well as on
the c h e m i c a l p r o p e r t i e s of t h e d r u g molecule.
T h e diffusivity of 5-ALA in t i s s u e s h a s n o t
been reported, but a reasonable estimate can
be o b t a i n e d by u s i n g t h e v a l u e for a compound with approximately the same molecular
w e i g h t (9, 10). T y p i c a l l y diffusion profiles for
glucose in a m e d i u m w i t h low diffusivity, eg
human
a o r t a l a o r t i c i n t i m a (diffusivity,
~:=1.7" 1 0 - 1 ~ 2 s - l ) , a r e s h o w n in Fig. 3(a).
T h e c o r r e s p o n d i n g profiles in t h e case of a
m e d i u m w i t h h i g h diffusivity, eg p l a s m a
( ~ : = 0 . 8 8 . 1 0 - 9 m 2 s - 1), a r e s h o w n in Fig. 3(b).
T h e c o r r e s p o n d i n g t i m e r e q u i r e d for t h e
in situ d r u g c o n c e n t r a t i o n to r e a c h 50% of
t h a t of t h e s u r f a c e l a y e r is s u m m a r i z e d in
T a b l e 1.
263
1
0.8
O9
xx ",.,,
0.6
~x
"~176
9~ 0.4
0.2
(a)
i
1
2
3
Depth (ram)
~
,
I
I
I
I
I
I
4
~:":.':::,:: ......
o.8
" " : ~ .............
......................
0.2 0
1
2
3
Depth (ram)
4
5
Fig. 3. Diffusion profiles (normalized density) after 1
(
), 5 ( - - - ) and 10 ( . . . ) h diffusion. (a)
K=1.7 9 10 -1~ m 2 s -1, (b) K ' = 0 . 8 8 9 10 - 9 m 2 s -1.
Table 1. Time required to reach 50% of the surface
drug concentration at various depths
Depth
(ram)
0.5
1.0
1.5
2.0
2.5
3.0
~:=0.88 910 - 9
(m2 s - 1)
K=1.7 - 10- lo
(m2 s - 1)
6 min
21 min
46 min
I h 26 min
2 h 16 min
3 h 10 min
28 rain
1 h 49 rain
4 h 5 min
7 h 3 min
11 h 46 min
15 h 16 min
T h e m o d e l r e s u l t s p r e s e n t e d in Fig. 3 a n d
Table I clearly show the depth dependence and
r a p i d i t y of t h e diffusion. T h e profile flattens
a n d t h e r a p i d i t y of p r o p a g a t i o n diminishes as
the d r u g p e n e t r a t e s m o r e deeply into t h e
m e d i u m (6) [the time, t (s), r e q u i r e d to r e a c h
50% of t h e s u r f a c e c o n c e n t r a t i o n at a
d e p t h • (m) c a n be a p p r o x i m a t e l y e x p r e s s e d by
t ~ x2•- 1, w h e r e K (m 2 s - 1) is t h e diffusivity].
T h e diffusion t i m e is s t r o n g l y d e p e n d e n t on
t h e p r o p e r t i e s of t h e tissue; t h e 50% v a l u e of
t h e profile will r e a c h d o w n to 3 m m d e p t h a f t e r
L.O. Svaasand, B.J. Tromberg, P. Wyss et al
264
3 h in a tissue with properties resembling those
of the serum, whereas it will require up to 15 h
in a tissue with a dense matrix such as the
aortal aortic intima.
Elevated pressure within tumours may
result in an outward fluid flow that could
counteract drug diffusion. The 'driving force'
of this transport mechanism is the pressure
gradient, and the convective flux of drug is
proportional to the pressure gradient and to
the percolative properties of the tissue. However, the transport of low molecular weight
hydrophilic or lipophilic solute molecules is
primarily determined by diffusion, whereas
convection might be of equal importance for
larger molecules (9).
A convective flow counteracting the diffusion process will also result in a steeper drug
distribution profile. The steady-state distribution, ie the distribution after an infinitely long
time, will not be constant throughout the tissue but will decay exponentially with distance
from the surface [the distribution will decay to
1/e=0.37 after a depth 5. . . . . . tive=Kq/RkAp
where q (Pas) is the viscosity of the solvent, R
is the ratio of the velocity of the solute to that
of the solvent, Ap (Pa m - l ) is the pressure
gradient and k (m2) is the constant characterizing the percolative properties of the tissue, ie
the Darcy constant].
The transport process will, however, always
be dominated by diffusion immediately after
the application, due to the very steep diffusion
profiles at t h a t time. The relative importance
of the counteracting convective flow increases
with time as the diffusion profile flattens,
and the process will be dominated by convection after a time, Tconvective, given by
Tconvective ~ ~2convective/K ).
The distribution of a photosensitizer is also
influenced by mechanisms such as clearance
by lymphatic flow or blood perfusion and by
conversion to other compounds, eg the conversion of 5-ALA to PpIX. These mechanisms will,
in the same manner as a counteracting convective flow, result in a drug distribution profile
that decays exponentially with depth (the distribution will decay to 1/e=0.37 after a depth
5clearance:~Tcl .......
where Tel . . . . . . .
is the
relaxation time, ie the time required for the
local concentration to deplete by a factor
of l/e).
The transport process will be dominated by
diffusion immediately after the application of
the drug, in the same manner as in the presence of convection. Drainage mechanisms will
0.8
0.6
0.4
0.2
I
0
i
,
,
1
,
,
I
2
,
r
,
,
I
3
,
,
,
r
I
,
, ,",
4
5
Depth (mm)
Fig. 4. S t e a d y - s t a t e distribution profile in p r e s e n c e of
clearance. - - ,
no clearance; - - - ,
high diffusivity; . . . .
low diffusivity. C l e a r a n c e time "Cclearance= 10 h (high diffusivity
~:=0.88 9 10 -9 m 2 s -1, low diffusivity K = 1 . 7 9 10 - l ~ m 2 s - l ) .
dominate at a time larger than the given
relaxation time. This phenomenon is illustrated in Fig. 4 for a relaxation time (clearance
time) of •clearance=10h under high and low
diffusivity conditions.
The steady-state drug distribution, which
represents the upper limit for the penetration,
is now reduced to 50% of surface levels at
depths of 1.7 and 3.9mm for low and high
diffusivity, respectively.
The intact stratum corneum will provide a
diffusion barrier for topically applied drugs.
This diffusion through a barrier is determined
by the step in drug concentration across the
barrier, and by a permeability, K (m s 1),
which characterizes the properties of the barrier for the specific molecule. The effect of the
barrier is maximum immediately after application of the drug, but the relative importance of
the barrier decreases with time (the diffusion
in tissue will represent the limiting factor after
the profile has reached a depth of 5barrier=K/K,
ie approximately after a time Tbarrier----K/K2).
Thus, irradiation should start as soon as
possible if full advantage of selective uptake is
to be taken in PDT of basal cell carcinoma
with a defective stratum corneum. Sufficient
time must, of course, be allowed for 5-ALA to
diffuse to the required depth and for PpIX to be
formed.
GENERATION AND PHOTOBLEACHING OF
PplX
Elevated concentrations of 5-ALA result in a
build-up of the active sensitizer PpIX. Optical
irradiation of PpIX initiates energy transfer
265
Topical Administration of Photosensitizers
to t h e c y t o t o x i c a g e n t , eg to s i n g l e t o x y g e n .
However, PpIX will also decompose during
i r r a d i a t i o n . T h e p h o t o p r o d u c t s c a n be p h o t o d y n a m i c a l l y i n a c t i v e , or some p r o d u c t s c a n be
a c t i v e a t o t h e r w a v e l e n g t h s . T h e d e c a y of t h e
a c t i v e c o m p o u n d c a n be c h a r a c t e r i z e d b y a
b l e a c h i n g p a r a m e t e r w h i c h c o r r e s p o n d s to t h e
fluence r e q u i r e d to r e d u c e t h e c o n c e n t r a t i o n
to 1/e=0.37 of t h e i n i t i a l v a l u e . T h e b l e a c h i n g
fluence of PpIX, m e a s u r e d in v i v o d u r i n g t r e a t m e n t of h u m a n b a s a l cell c a r c i n o m a , h a s b e e n
r e p o r t e d to be 17.2 J cm 2 a t 635 n m (11). T h i s
b l e a c h i n g f l u e n c e will, w h e n c o r r e c t e d for t h e
e l e v a t e d e p i d e r m a l fluence, c o r r e s p o n d to a n
in s i t u b l e a c h i n g f l u e n c e in t h e r a n g e 0=40 J
cm - 2 a n d 0 = 2 0 J cm - 2 for C h i n e s e h a m s t e r
o v a r i a n cells (CHO cells) a n d h u m a n e p i t h e l i u m c a r c i n o m a cells (HEC cells), r e s p e c t i v e l y
(12).
T h e p h e n o m e n o n of b l e a c h i n g l i m i t s t h e
m a x i m u m a m o u n t of s i n g l e t o x y g e n t h a t c a n
be g e n e r a t e d . T h e c o n c e n t r a t i o n of t h e a c t i v e
p h o t o s e n s i t i z e r is r e d u c e d to 1/e2~0.1 a f t e r
e x p o s u r e to a n o p t i c a l dose t w i c e as l a r g e as
t h e b l e a c h i n g fluence. Thus, a b o u t 90% of t h e
m a x i m u m a m o u n t of s i n g l e t o x y g e n t h a t c a n
be g e n e r a t e d is o b t a i n e d w h e n t h e o p t i c a l dose
r e a c h e s t w i c e t h e b l e a c h i n g fluence. A b l e a c h i n g fiuence for P p I X in t h e r a n g e 20-50 J cm 2
t h u s i n d i c a t e s t h a t a n y i n c r e a s e of t h e o p t i c a l
dose a b o v e 100 J c m - 2 m a y h a v e no c l i n i c a l
relevance.
CONCLUSION
T h e r e s u l t s i n d i c a t e t h a t t h e l i m i t i n g f a c t o r in
P D T w i t h t o p i c a l d r u g a p p l i c a t i o n w i l l be
t r a n s p o r t of t h e d r u g i n t o t h e t u m o u r , r a t h e r
t h a n d e p l e t i o n of t h e o p t i c a l fluence. T h e t i m e
r e q u i r e d for s m a l l m o l e c u l a r w e i g h t p h o t o s e n s i t i z e r s , s u c h as 5-ALA, to diffuse to a d e p t h of
2.5-3 mm m a y r a n g e from 3 to 15 h.
The m a x i m u m effective in s i t u o p t i c a l dose
at 630-635 n m is l i m i t e d b y t h e b l e a c h i n g flu-2
ence of P p I X to a p p r o x i m a t e l y 100 J cm
This o p t i c a l dose c a n be d e l i v e r e d to a d e p t h of
m o r e t h a n 3 m m by i r r a d i a t i n g t h e s u r f a c e
w i t h 100 m W cm - 2 for 20 min. E x c i t a t i o n of
the photosensitizer with green light might also
r e p r e s e n t a n i n t e r e s t i n g o p t i o n in t h e t r e a t m e n t of s u p e r f i c i a l l e s i o n s , s i n c e t h e a p p l i c a t i o n of 100 m W c m - 2 at 514 n m for 20 m i n w i l l
p r o v i d e a n in s i t u o p t i c a l dose of 100 J cm 2 to
a d e p t h of m o r e t h a n 0.5 mm.
ACKNOWLEDGEMENTS
This work was made possible, in part, through access to
the Laser Microbeam and Medical Program (LAMMP) and
the Clinical Cancer Center Optical Biology Shared
Resource at the University of California, Irvine. These
facilities are supported by the National Institute of Health
under grants RR-O01192 and CA-62203, respectively.
Beckman Laser Institute programmatic support was
provided by the Department of Energy (DOE#DE-FG0391ER61227), and the Office of Naval Research
(ONR#N00014-91-C-O134). Additional funding was provided by the National Institute of Health (GM-50958) and
the Academ. Nachwuchsfoerderung, University of Ziirich.
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(submitted)
words: Photodynamic
therapy; Dosimetry; 5Aminolaevulinic acid; Protoporphyrin IX; Photobleaching; Topical application
Key