Deposition of viprostol (a synthetic PGE2 vasodilator) in the skin following topical administration to laboratory animals

Malathion dermal permeability in relation to dermal load: Assessment by physiologically based pharmacokinetic modeling of in vivo human data

Estimates of dermal permeability (K_p), obtained by fitting an updated human PBPK model for malathion to previously reported data on excreted urinary metabolites after 29 volunteers were dermally exposed to measured values of [¹⁴C]malathion dermal load (L), were used to examine the empirical relationship between K_p and L. The PBPK model was adapted from previously reported human biokinetic and PBPK models for malathion, fit to previously reported urinary excretion data after oral [¹⁴C]malathion intake by volunteers, and then augmented to incorporate a standard K_p approach to modeling dermal-uptake kinetics. Good to excellent PBPK-model fits were obtained to all of 29 sets of cumulative urinary metabolite-excretion data (ave. [±1 SD] R² = 0.953 [±0.064]). Contrary to the assumption that K_p and L are independent typically applied for dermally administered liquids or solutions, the 29 PBPK-based estimates of K_p obtained for malathion exhibit a strong positive association with the 2/3rds power of L (log-log Pearson correlation = 0.925, p = ∼0). Possible explanations of this observation involving physico-chemical characteristics and/or in vivo cutaneous effects of malathion are discussed. The PBPK model presented, and our observation that K_p estimates obtained by fitting this model to human experimental urinary-excretion data correlate well with L^2/3, allow more realistic assessments of absorbed and metabolized dose during or after a variety of scenarios involving actual or potential dermal or multi-route malathion exposures, including for pesticide workers or farmers who apply malathion to crops.

Prostaglandin-induced hair growth

Latanoprost, used clinically in the treatment of glaucoma, induces growth of lashes and ancillary hairs around the eyelids. Manifestations include greater thickness and length of lashes, additional lash rows, conversion of vellus to terminal hairs in canthal areas as well as in regions adjacent to lash rows. In conjunction with increased growth, increased pigmentation occurs. Vellus hairs of the lower eyelids also undergo increased growth and pigmentation. Brief latanoprost therapy for 2-17 days (3-25.5 microg total dosage) induced findings comparable to chronic therapy in five patients. Latanoprost reversed alopecia of the eyelashes in one patient. Laboratory experiments with latanoprost have demonstrated stimulation of hair growth in mice and in the balding scalp of the stumptailed macaque, a primate that demonstrates androgenetic alopecia. The increased number of visible lashes is consistent with the ability of latanoprost to induce anagen (the growth phase) in telogen (resting) follicles while inducing hypertrophic changes in the involved follicles. The increased length of lashes is consistent with the ability of latanoprost to prolong the anagen phase of the hair cycle. Correlation with laboratory studies suggests that initiation and completion of latanoprost hair growth effects occur very early in anagen and the likely target is the dermal papilla.

Skin distribution of fipronil by microautoradiography following topical administration to the beagle dog

To investigate the localisation of fipronil in dog skin, [14C]-fipronil was topically applied to a male beagle dog (spot-on administration) at the therapeutic dose of 10 mg/kg. By means of autohistoradiography, the radioactivity was precisely detected in the skin and appendages at various intervals after application. Radioactivity was predominantly observed within the stratum corneum, the viable epidermis, and in the pilo-sebaceous units (mainly in the sebaceous glands and epithelial layers). [14C]-fipronil was significantly detected in these structures up to 56 days post-treatment, in the application zone (neck) but also in the lumbar zone, thus indicating the mechanical displacement of fipronil. No radioactivity was detected in either the dermal or the hypodermal layers, confirming the low percutaneous passage of fipronil.

Transfollicular drug delivery

The hair follicle, hair shaft, and sebaceous gland collectively form what is recognized as the pilosebaceous unit. This complex, three-dimensional structure within the skin possesses a unique biochemistry, metabolism and immunology. Recent studies have focused on the hair follicle as a potential pathway for both localized and systemic drug delivery. Greater understanding of the structure and function of the hair follicle may facilitate rational design of drug formulations to target follicular delivery. Targeted drug delivery may enhance current therapeutic approaches to treating diseases of follicular origin. Presented here is a review of follicular drug delivery and a discussion of the feasibility of the pilosebaceous unit as a target site.

Follicles play an important role in percutaneous absorption

The relative importance of stratum corneum and follicles in percutaneous absorption is not fully understood. In order to quantitatively investigate the importance of the transappendageal route, we have previously developed a model of skin without follicles regrown dorsally on the hairless rat. Percutaneous absorption was compared, using a diffusion cell, in appendage-free skin relative to normal skin, and a predominant role of follicles for in vitro diffusion of [3H]hydrocortisone was noted. Results presented here of in vitro diffusion of tritiated hydrocortisone, niflumic acid, caffeine, and p-aminobenzoic acid, applied in acetone, confirm that appendageal diffusion is the major pathway in hairless rat skin. In the absence of follicles, the steady-state flux and the amounts diffusing in 24 or 48 h are 2-4 times lower than in normal skin. These results were confirmed in a second model in which diffusion of [3H]hydrocortisone was studied on skin samples taken one day after birth, at which time rat skin is still devoid of follicles, relative to five-day postnatal skin samples, in which follicles are fully developed. The steady-state flux and the total diffusion in 24 h were fivefold lower in follicle-free skin. These results support the view that follicles may have a far greater importance in percutaneous absorption than is generally assumed.

Pre-systemic metabolism of viprostol in the monkey following oral and topical administration

1. 14C-Viprostol, (I), a synthetic PGE2 analogue, was administered to 6 monkeys, orally, topically and intravenously in a three way crossover study. Total radioactivity and the pharmacologically active acid formed by rapid hydrolysis of viprostol in vivo, (II), were measured in plasma to determine absorption and absolute bioavailability. 2. After oral dosing approx. 31% of drug-related radioactivity was absorbed. Systemic bioavailability of unchanged active acid was only 7.3%. This indicates significant first-pass metabolism after oral administration, since only 23% of the absorbed radioactivity was available as 'unchanged' active drug (II). 3. After topical dosing, transdermal absorption of total radioactivity by 48 h averaged only 5% of dose. Absolute bioavailability of II averaged 3.8% of dose. This indicates that after transdermal absorption 74% of the absorbed radioactivity was available systemically as the active acid II, with the remainder being subject to pre-systemic metabolism.