Pesticide Formulations - American Chemical Society

Chapter 10

Structure—Penetration Relationships in Percutaneous Absorption G. Ridout and R. H. Guy

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Department of Pharmacy, University of California, San Francisco, CA 94143 Insight into the physicochemical factors that control percutaneous absorption (PA) i s necessary for prediction of the dermal penetration c h a r a c t e r i s t i c s of environmental and i n d u s t r i a l toxinogens that contact the skin. Penetration data for series of barbiturates) nicotinates, phenols, steroids and a selection of other compounds were obtained from our laboratory and from the l i t e r a t u r e ; experiments using (a) excised skin (hairless mouse or human cadaver) and (b) artificial membranes (dimethylpolysiloxane or model lipid systems representative of those present i n the stratum corneum) were considered. The rank order of resistance to penetration provided by the model systems matched that of excised skin. The transport data were correlated with several corresponding organic-aqueous p a r t i t i o n c o e f f i c i e n t s (K). In most cases, membrane permeability increased with increasing K. However, exceptions were found and the degree of correlation was dependent upon the organic phase of the p a r t i t i o n i n g system and/or the physicochemical nature of the penetrants studied. Although K(octanol-aqueous) has been widely employed i n the past, other l i p i d s such as isopropyl myristate or tetradecane may provide more relevant media for the assessment of penetrant l i p o p h i l i c i t y i n structure-PA correlations. Human skin i s a multilayered heterogeneous organ composed of two main tissue layers, the dermis and epidermis, supported on a layer of subcutaneous f a t (Figure 1). The dermis consists of a matrix of dense collagenous, e l a s t i c connective tissue imbedded i n mucopolysaccharide. I t has an average thickness of 3-5 mm and forms the bulk of the skin (1). The main functions of the dermis are to support and bind the epidermis and to impart strength and e l a s t i c i t y to the skin. Many structures are distributed throughout, and supported by, the dermis, including the vascular and nerve supplies, 0097-6156/88/0371-0112$06.00/0 ° 1988 American Chemical Society


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hair f o l l i c l e s and sebaceous and sweat glands. The avascular epidermis i s the most s u p e r f i c i a l skin layer and i s formed of several layers of rapidly p r o l i f e r a t i n g c e l l s (keratinocytes) which ascend from the basal layer at the epidermal-dermal junction. The thickness varies; the average being 40-50 ym over most of the body surface (2). As the c e l l s migrate upwards they generate fibrous proteins and s p e c i f i c l i p i d s and transform from l i v i n g c e l l s into a dead, keratinised zone that i s ten to f i f t e e n c e l l layers deep. I t is generally accepted that this outermost skin layer, the stratum corneum, and more s p e c i f i c a l l y the i n t e r c e l l u l a r l i p i d matrix within, i s the rate l i m i t i n g b a r r i e r to the transport of most molecules across the skin (3). Most simply, therefore, skin provides a bilaminate membrane barrier to transport consisting of the thin, but highly resistant and l i p o p h i l i c stratum corneum and the more substantial, aqueous g e l - l i k e viable tissue (the l i v i n g epidermis and viable tissue) ( 4 ) . The long term objective of this research i s to predict the k i n e t i c s and extent of percutaneous penetration i n vivo from the physicochemical and pharmacokinetic properties of the chemical i n question. From this knowledge, one should for example, be able to determine the r i s k of t o x i c i t y a r i s i n g from dermal exposure to pesticide formulations. As a f i r s t step, penetration data f o r series of barbiturates, nicotinates, phenols, steroids and a selection of other compounds were obtained from our laboratory and from the l i t e r a t u r e ; experiments using (a) excised skin (hairless mouse or human cadaver) and (b) a r i f i c i a l membranes (dimethylpolysiloxane or model l i p i d systems) were considered. P r i n c i p a l attention was focussed upon the u t i l i t y of various organic-aqueous p a r t i t i o n c o e f f i c i e n t s (K) as rank order indicators of transdermal f l u x and upon the p r e d i c t a b i l i t y of the d i f f e r e n t model systems investigated. Systems Studies [1] The rotating d i f f u s i o n c e l l (RDC) has been widely used to study the transport of various penetrants from aqueous solution through a r t i f i c i a l l i p i d membranes (5-6). The model membrane consists of a 0.2 urn c e l l u l o s e n i t r a t e membrane f i l t e r saturated with the chosen l i p i d . In the RDC, stagnant d i f f u s i o n layers of thickness Z are created on either side of the l i p i d membrane. The c e l l can be rotated at d i f f e r e n t speeds (W) and, once pseudo-steady state conditions have been established, theory predicts that Z i s proportional to W ' ( 5 ) . Permeability c o e f f i c i e n t s (P) are measured as a function of W ' and extrapolation of the resultant l i n e a r relationship to i n f i n i t e rotation speed (where Zp i s zero) yields the i n t r i n s i c t o t a l membrane permeability to solute ( P ) . P* i s a composite permeability c o e f f i c i e n t that accounts for permeation across the membrane and the two aqueous-organic i n t e r f a c e s . The u t i l i t y of the RDC as a system i n which to set up such an in v i t r o model f o r percutaneous absorption has been studied previously using an isopropy1 myristate (IPM) membrane as the stratum corneum model (5-6). Most recently, Hadgraft and Ridout (1987) have measured the permeability c h a r a c t e r i s t i c s of an IPM membrane to eight model penetrants and constrasted these with steady-state permeation through excised f u l l - t h i c k n e s s human cadaver n

n

v

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PESTICIDE FORMULATIONS: INNOVATIONS AND DEVELOPMENTS

skin (7). The model chemicals were selected so as to r e f l e c t the diverse physicochemical properties that a penetrant might possess* They were an analogous series of barbiturates (amylobarbitone [A], barbitone [B], butobarbitone [U] and phenobarbitone [P]), two bases (isoquinoline [1] and nicotine [N]), hydrocortisone [H] and s a l i c y l i c acid [S]. The mean values of P for each penetrant crossing the l i p i d membrane were determined i n at least f i v e separate experiments* Figure 2 compares the b i o l o g i c a l barrier with the IPM membrane using K(IPM-aqueous) [K.] as the index of lipophilicity* If the membrane i s considered homogeneous and i t i s assumed that the d i f f u s i o n c o e f f i c i e n t s across the membrane are invarient compared to K, with respect to the low molecular weight solutes studied, then P should be a l i n e a r function of K; the RDC data f i t s this relationship w e l l . However, although the skin data 8hows a s i m i l a r trend, the permeation of nicotine i s r e l a t i v e l y high, a fact which the model f a i l s to predict. In addition, the o v e r a l l permeability of the model membrane i s 100-fold higher than that of skin. Tetradecane (TD) has been suggested as being a more representative model of stratum corneum l i p i d s (6). The permeability of a TD membrane to the same eight penetrants was measured and the correlation with human skin reassessed using K(TDaqueous) [K ] as the l i p o h i l i c i t y index (Hadgraft, J . ; Ridout, 6. Int. J . Pharm., i n press). Figure 3 shows that TD does predict the increased permeation of nicotine and that the magnitude of permeability for this membrane i s only 100-fold higher than the skin. TD provides, therefore, a better model than IPM for this set of compounds. In Figure 4 the two sets of RDC data are shown on a common scale of K. Although the upper l i m i t of K i s similar for both l i p i d s , the range of i s much condensed i n the hydrophilic region compared with K * I t follows that, IPM has a greater capacity for uptake of hydrophilic chemicals than TD. Figure 5 shows the three sets of permeability data plotted as a function of penetrant K(octanol-aqueous) [ K ] , the t r a d i t i o n a l l y used index of l i p o p h i l i c i t y . The e s s e n t i a l l y linear relationships between permeability and l i p o p h i l i c i t y observed using both and K are less apparent, suggesting that octanol i s a poorer representative for stratum corneum than IPM or TD. The range of K covers 2 log units (similar to K^); however, there i s an o v e r a l l s h i f t to the l i p o p h i l i c end of the scale. With the exception of the barbiturate s e r i e s , the rank order of K i s s i g n i f i c a n t l y d i f f e r e n t for each l i p i d used. Guy and co-workers (Houk, J . ; Guy, R. H. Chem. Rev., i n press; Ridout, G.; Houk, J . ; Palmier!, J . A.; A l i a b a l d i , D.; Guy, R. H., unpublished data) have also employed the RDC with IPM and TD membranes to study the penetration c h a r a c t e r i s t i c s of series of nicotinates, phenols and steroids. Figures 6 and 7 show the relationship between P' and the corresponding K for IPM and TD membranes, respectively. The data also includes those of Hadgraft and Ridout, described above. In both cases, a parabolic dependency of P' upon K was found, suggesting that as degree of l i p o p h i l i c i t y increases from a value of between 1 and 2, the rate-controlling step i n permeation s h i f t s from membrane d i f f u s i o n to i n t e r f a c i a l transport. As previously, the range of K for IPM i s compressed and shifted to the l i p o h i l i c end of the scale.


f

fc

t

Q

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Q



RIDOUT AND GUY

LogKi

Figure 2.

Permeability c o e f f i c i e n t s of a diverse set of compounds through an IPM membrane ( • ) and excised human skin (•) plotted as a function of penetrant K . t

Key:

A P H N

* •

amylobarbitone; B - barbitone; phenobarbitone; U • butobarbitone; hydrocortisone; I * isoquinoline; nicotine; S • s a l i c y l i c a c i d .

116


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Figure 3.

Permeability c o e f f i c i e n t s of a diverse set of compounds through an IPM membrane (•) and excised human skin (•) plotted as a function of penetrant K « Compound i d e n t i t i e s as Figure 2. t

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Figure 4.

Permeability c o e f f i c i e n t s of a diverse set of compounds through both IPM (•) and TD (•) membranes plotted as a function of penetrant K.

RIDOUT AND GUY


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Figure 5*

Permeability c o e f f i c i e n t s of a diverse set of compounds through both IPM (•) and TD (•) membranes and excised human skin (•) plotted as a function of penetrant K . Q

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Figure 6.

Permeability and steroids (•) through of penetrant

c o e f f i c i e n t s of nicotinates, phenols (•) and a diverse set of compounds an IPM membrane plotted as a function K,.

117


118


Figure 7»

Permeability c o e f f i c i e n t s of nicotinates, phenols and steroids (•) and a diverse set of compounds (•) through a TD membrane plotted as a function of penetrant K..

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[2] The i n v i t r o permeability of h a i r l e s s mouse skin (8) and human epidermis (9) to series of phenols has been studied. Figure 8 compares this data with that obtained by Houk and Guy, described above. K was used as the l i p o p h i l i c i t y scale as neither nor K was available for a l l the compounds studied. As previously, i t i s evident that the a r t i f i c i a l TD membrane can provide a reasonable model of human skin, the permeability being 10-fold lower than that of the IPM membrane. The permeability of murine skin i s compared with that of human skin for s i x of the phenols i n Figure 9. I t i s evident from the limited data a v a i l a b l e , that murine skin provides a barrier which i s less than an order of magnitude smaller than that of human skin. Pesticide Formulations Downloaded from pubs.acs.org by CORNELL UNIV on 10/11/16. For personal use only.

Q

t

[3] Dimethylpolysiloxane ( S i l a s t i c ) membranes have also been used to model chemical transport across skin. The permeability of dimethylpolysiloxane to a series of phenols (10) and to homologous series of a l k y l para-aminobenzoates (11) as a function of K(hexaneaqueous) [ K l Is shown i n Figure 10. The penetrants studied were phenol (PH), dinitrophenol (DN), 2-nitrophenol (2N) and 4nitrophenol (4N); and the methyl (ME), ethyl (ET), propyl (PR), butyl (BU) and pentyl (PE) aminobenzoates• Once again, l i n e a r relationship was found. I t appears that, K ( l i k e K ) offers a reasonable index for membrane l i p o p h i l i c i t y . n e x

n e x

fc

[4] The permeabilities of several steroids through excised human epidermis and the corresponding K(stratum corneum-aqueous) [ K 1 were reported by Scheuplein (12). Figure 11 shows the resultant l i n e a r relationship between P and K ( r • 0*86). When P was plotted against K , on the other hand, the c o r r e l a t i o n was less impressive, r - 0.69. Scheuplein also determined K(amyl caproateaqueous) [K ] and K(hexadecane-aqueous) [ K j j ] for each of the steroids. Of these, amyl caproate provided a s i m i l a r index of l i p o p h i l i c i t y to K (Figure 12, r - 0.87), as compared to either hdec ^ " ) * » should be noted that the slope of log P versus log K was approximately twice that observed when the model l i p i d p a r t i t i o n c o e f f i c i e n t s were used. Therefore, although these simple systems are useful rank order predictors, their quantitative u t i l i t y requires further careful consideration. 8C

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Summary Although K has been widely employed i n the past, other l i p i d s such as esters (amyl caproate, isopropyl myristate) or alkanes (hexane, tetradecane) may provide more relevant media for the assessment of penetrant l i p o p h i l i c i t y i n structure-PA c o r r e l a t i o n s . The rotating d i f f u s i o n c e l l , with appropriate l i p i d membranes, and h a i r l e s s mouse skin, under c a r e f u l l y controlled conditions, can o f f e r reasonable models for skin penetration studies. A p r a c t i c a l , and inexpensive, means of assessing the r e l a t i v e r i s k of t o x i c i t y a r i s i n g from dermal exposure i s suggested by the results described i n this paper. For a s t r u c t u r a l l y related series of chemicals, measurement of (a) a simple lipid-water p a r t i t i o n c o e f f i c i e n t and (b) selected permeabilities through a model membrane composed of the l i p i d , can accurately rank order transport rates across the skin. Such an approach should permit, therefore, Q


120

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-1


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Figure 8.

Comparison between the permeability c o e f f i c i e n t s of phenols through excised human ( • ) and mouse skin (O) and IPM (•) and TD (•) membranes*

-0.6 n

-1.0-

-2.4

-2.0 Log [P human (cnVh)]

Figure 9«

Comparison between the permeability c o e f f i c i e n t s of chlorocresol (CC), 2-chlorophenol (2C), dichlorophenol [DC], 4-nitrophenol (4N), phenol (PH) and trichlorophenol (TC) through excised human and mouse s k i n .

RIDOUT AND GUY


1n


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1 - 2

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Figure 10*

Permeability c o e f f i c i e n t s of a l k y l paraaminobenzoates at 25°C (•) and phenols at 37°C ( • ) through a dimethylpolysilaxane membrane as a function of K,

0.6

1.0

1.4

1.8

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Figure 11*

Permeability c o e f f i c i e n t s of a series steroids through excised human epidermis as a function of K (linear regression l i n e drawn through the data)* g c


122 PESTICIDE FORMULATIONS: INNOVATIONS AND DEVELOPMENTS

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123

selection of a chemical which demonstrates high potency f o r i t s proposed u t i l i z a t i o n (e.g., as a p e s t i c i d e ) , but presents a low r i s k with respect to worker t o x i c i t y resulting from dermal exposure* Acknowledgments


Financial support was provided by grants from NIOSH, NI6MS, EPA, SERC and the UC Toxic Substances Program. We thank Joy Houk, Darius Aliabadi, and James Palmier! for sharing with us their unpublished data, and Professor J* Hadgraft f o r helpful comment* Literature 1.

2. 3. 4. 5. 6. 7. 8.

9. 10.

11.

12.

Cited

Katz, M.; Poulsen, B. J . In Handbook of Experimental Pharmacology; V o l . 28, Pt. 1; Bodie B. B.; G i l l e t t e , J . , Eds.; Springer-Verlag: B e r l i n , 1971; pp. 102-74. Morley, S. M., Gaylarde, P. M.; Sarkany, I. C l i n . E x p t l . Dermatol. 1985, 10, 51-57. Hadgraft, J. Int. J. Pharm. 1983, 16, 255-270. Scheuplein, R. J . J . Invest. Dermatol. 1967, 48, 79-88. Albery, W. J . ; Hadgraft, J . J . Pharm. Pharmacol. 1979, 31, 65-68. Guy, R. H.; Fleming, R. Int. J. Pharm. 1979, 3, 143-50. Hadgraft, J . ; Ridout, G. Int. J. Pharm. 1987, 39, 149-56. Huq, A. S.; Ho, N. F. H.; Husari, N.; Flynn, G. L.; Jetzer, W. E.; Condie, L.; Arch. Environ. Contam. Toxicol. 1986, 15, 557-66. Roberts, M. S.; Anderson, R. A.; Swarbrick, J . J . Pharm. Pharmacol. 1977, 29, 667-83. Jetzer, W. E.; Huq, A. S.; Ho, N. F. H.; Flynn, G. L.; Duraiswamy, N.; Condie, L. J . Pharm. S c i . 1986, 75, 10981103. Flynn, G. L. In Percutaneous Absorption; Bronaugh, R. L.; Maibach, H. I., Eds.; Marcel Dekker: New York, 1985; pp. 1742. Scheuplein, R. J . J . Invest. Dermatol. 1969, 52, 63-70.

RECEIVED January 22, 1988

Pesticide Formulations - American Chemical Society

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