DRUG PERMEATION THROUGH SKIN

Mario Grassi

Department of Chemical Engineering (DICAMP)UINVERSITY OF TRIESTE

1 - SKIN STRUCTURE

non-living layer of keratin-filled cells surrounded by a lipid-rich extracellular matrix

viable tissue devoid of blood vessels.

contains capillary loops

Adapted from M. Prausnitz et al. NATURE REVIEWS, DRUG DISCOVERY, 3, (2004),115

2 – PERMEATION ROUTES

Sweat pore

1 2 3

Hair

Dermis

Hair follicle

Sebaceous gland

Epidermis

Stratum Corneum

Sweat gland

Sweat duct

Adapted from B. W. Barry, Adv. Drug. Del. Review, 54, (2002), S31-40

3 – EXPERIMENTAL SET UP 1

donor

receiver

water jacket

sampling port

magnetic stirrer

skin

water IN

water OUT

donor

Acyclovir solid particles

stratum corneum

dermis + epidermis

stagnant layer

receiver

X = 0

X = h sc

X = h sc+ h de

X = h sc+ h de+ h ss

4 – MODELLING 1

DRUG DISSOLUTION

DRUG DIFFUSION

DRUG CONCENTRATION INCREASE

Cd = Cd0

M = M0

Css = 0

Cde = 0

Css = 0

Cr = 0

Drug

X

CD

tt

C scscsc

X

CD

tt

C dede

de

X

CD

tt

C ssssss

S. Corneum

Dermis + Epidermis

stagnant layer

FICK LAW

donor

Acyclovir solid particles

stratum corneum

dermis + epidermis

stagnant layer

receiver

X = 0

X = h sc

X = h sc+ h de

X = h sc+ h de+ h ss

0

1wdpDPd

dd

Xs X

CSDCCSKV

t

CV

scsc hX

de

hX

sc

X

CD

X

CD

desc

descdesc hhX

ss

hhX X

CD

X

CD

ssde

de

ssdesc hhhX

sc

X

CSD

t

CV

scr

r

d

ssp1 C

CK

sc

dep2 C

CK

ss

dep3 C

CK

r

ssp4 C

CK

1

0

3m1rrddd0d0

3m1

m1

m1

1

dddh hhh

hh

hh

h

XSCXSCXSCCVCVMCVM

SOLID SURFACE VARIATION: MONODISPERSED

PARTICLES SYSTEM

30

0

4

3p R

MN

SOLID DRUG

M N M N R p p p34

3

R RM

M 0

0

3

Mass balance

20P0 4 RNA 0

0

3

AR

Particles initial surface area

SAMPLING TECNIQUE

donor

receiver

water jacket

sampling port

magnetic stirrer

skin

water IN

water OUT r

rrbr

ar V

VVCC

receiver drug concentration just before sampling.b

rC

arC

receiver drug concentration just after sampling.

CASE STUDY: ACYCLOVIR PERMEATION THROUGH RAT SKIN

ACYCLOVIR- Recordati , Milano- ANTIVIRAL, WHITE CRYSTALLINE POWDER (R = 5.7 m)- SURFACE AREA = 3370 cm2/g (mercury porosimeter)- U.V. PEAK ABSORBANCE 251 nm- SOLUBILITY in PBS (pH = 7.4; 37°C) Cs = 2.62 mg/cm3

- ACYCLOVIR DIFF. COEFF. (PBS, 37°C) Dss = 7.8*10-6 cm2/s (IDR)

- ACYCLOVIR DISS. CONST. (PBS, 37°C) KD = 5*10-5 cm/s

RAT SKIN:-Male hairless rats (Rnu eutimic, Charles River, MI, Italy) 5–7 weeks oldFull-thickness skin removed from abdomen by incision of the outermost layer with a surgical bisturi. Stratum corneum was separated from the dermis–epidermis by placing the full-thickness skin (dermis-side down) on a filter paper saturated with a 1% trypsin) solution at the temperature of 37 ◦C for 4 h.

STAGNANT LAYER THICKNESS: hss

Sh = f Rem Scn Sh = d/hss Sc = /( D) Re = d2/ = fluid densityD = drug diff. coeff.

d = stirrer diameter = fluid viscosity.

donor

receiver

water jacket

sampling port

magnetic stirrer

skin

water IN

water OUT

Dissolution

T = 25°C = 388 rpm = 505 rpm = 605 rpm

hss1 ==> Sh1

hss2 ==> Sh2

hss3 ==> Sh3

T = 37°C = 388 rpm = 505 rpm = 605 rpm

hss4 ==> Sh4

hss5 ==> Sh5

hss5 ==> Sh6

t

Vh

SD

sseCC R

W

1S

0

20

40

60

80

100

120

140

160

300 400 500 600 700(rpm)

Sh

(-)

Sh modelSh experimental

f = 3.2*10-6

m = 1.2n = 0.95

Sc = 1147Re = 5737

hss = 0.011 cm

PARTITION COEFFICIENTS: Kp1,2,3,4

The full skin/Acyclovir solution and the one-layer skin/Acyclovir solution partition coefficients Kskin and Kde are determined by immersion of both fragments in the Acyclovir solution (PBS pH 7.4) for 4 h at 37°C

Kskin =0.547Kde =0.95

Kp1 =0.5(literature)

Kp3 = Kp2*Kp1 Kp2 = Kskin*(1+G)-Kp1/(G*Kp1)

G = hsd/hsc= 11.7

Kp1 = 0.5 Kp2 = 1.1

Kp3 = 0.55 Kp4 = 1

0

50

100

150

200

250

300

0 2 4 6 8 10t(h)

Cr( g

/cm

3 )FIVE WEEKS OLD RATS: ONE LAYER (DERMIS-EPIDERMIS)

5 – RESULTS

D = 2*10-6 cm2/s

D = 2*10-6 cm2/s

D = 1*10-6 cm2/s

DAV = (1.7 ± 0.6)*10-6 cm2/s

Data (symbols) not corrected for dilution

Lines: model best fitting

SEVEN WEEKS OLD RATS: ONE LAYER (DERMIS-EPIDERMIS)

0

10

20

30

40

50

60

70

80

90

0 2 4 6 8 10t(h)

Cr( g

/cm

3 )

DAV = (8 ± 2)*10-7 cm2/s

D = 9.5*10-7 cm2/s

D = 6.5*10-7 cm2/s

Data (symbols) not corrected for dilution

Lines: model best fitting

FIVE WEEKS OLD RATS: FULL SKIN

0

0.5

1

1.5

2

2.5

3

0 2 4 6 8 10t(h)

Cr( g

/cm

3 )

DAV = (1.33 ± 1)*10-9 cm2/s

D = 2.5*10-9 cm2/s

D = 1*10-9 cm2/s

D = *10-10 cm2/s

Data (symbols) not corrected for dilution

Lines: model best fitting

SEVEN WEEKS OLD RAT: FULL SKIN

0

0.2

0.4

0.6

0.8

1

1.2

0 2 4 6 8 10t(h)

Cr( g

/cm

3 )

D = 6.5*10-10 cm2/s

Data (symbols) not corrected for dilution

Lines: model best fitting

COMMENTS

1Regardless animal age, stratum corneum represents the main barrier to drug permeation (Dde 1000 Dsc)

2 Skin permeability seems to decrease with age

3A considerable inter-animal variability is observed

4Model simulations reveal that pseudo steady state conditions are met after 2 hours for young animals (one layer skin), while they are met after 6 hours in the remaining cases

6 – COMPARISON THIS APPROACH / COMMON APPROACH 2

1

0

ir

r

rnr

nrc

n

i

CV

VCC

DATA CORRECTION FOR DILUTION

nrcC = corrected drug concentration after “n” samplings

nrC = experimental drug concentration after “n” samplings

irC = experimental drug concentration after “i” samplings

rV = sampling volume rV = receiver volume

00r C

ASYMPTOTIC FICK EQUATION SOLUTION

C mt qr m

S K C D

V h

p m

r

0q

S K C h

V

p

r

0

6

1) Pseudo steady state conditions

2) Cd = C0; Cr 0 (sink conditions)

3) Stagnant layer is neglected

DmV h

S K Cm

r

p

0

Dm = drug diff coeff through membrane

h = membrane thicknessS = membrane areaKp = partition coefficient

C mt qr

ASSUMING “m” AND “q” AS INDEPENDENT FITTING PARMETERS:

IS FITTED ON EXPERIMENTAL DATA TO GET Dm

FROM DIFFUSION COEFFICIENT TO RESISTANCE

1ss

ss

dede

de

p1sc

sco *D

h

KD

h

KD

hR

p12sc

sc

sc

o2

2

sc

oR0

sc

scsc KD

h

D

R

D

R DDD

THIS APPROACH

TRADITIONAL APPROACH

pmm KD

hR

p2mm

m22

m

mR

m

mmm KD

h

D

R

D

R DDD

Experiment number identifier

N

Animal age

(weeks)

F-value

Ro (s/cm) Rm (s/cm)

Dermis + epidermis (one layer skin)

1 5 732 (5.9 0.014)*104 (6.1 0.016)*104

2 5 261 (4.9 0.001)*104 (5.5 0.012)*104

3 5 136 (12.6 0.06)*104 (12.1 0.059)*104

4 7 864 (21.1 1.62)*104 (19.2 1.35)*104

5 7 5906 (15.6 0.814)*104

(15.0 0.765)*104

Full skin

6 5 595 (4.83 0.75)*106 (2.49 0.175)*106

7 5 45 (26.4 24.7)*106 (10.7 2.96)*106

8 5 99965 (13.7 6.7)*106 (7.68 0.014)*106

9 7 57 (12.7 5.2)*106 (5.76 0.13)*106

This approach Traditional approach

7 – REFERENCES

1) N. Coceani, I. Colombo, M. Grassi, Int. J. Pharm. 254 (2003) 197 –210.

2) Chien, Y.W. (Ed.), 1987. Transdermal Controlled Systemic Medications. Marcell Dekker, Inc., New York, Basel

(Chapter 2).