REVIEW OF INVESTIGATIONS OF DELIVERY VIA THE FOLLICULAR PATHWAY
Scheuplein in 1967 (42) did the mathematical modeling for follicular delivery of small molecular weight electrolytes and nonelectrolytes. He used reported values of thickness, diffusion constant, area of each layer of the skin. According to him, during the initial part of the experiment, the shunt pathway (follicular and sweat glands) is dominant and that introduces the lag period. At steady state, the transe- pidermal route is dominant. The point at which the shunt pathway becomes equal to the bulk pathway is around 300 seconds, as per these calculations. According to the paper, the concentration levels in the neighborhood of the ducts and the follicles may be very much higher than in the bulk of the epidermis in the early stages of diffusion, before steady-state diffusion is achieved for small molecules (ordinarily within the first one hour after application of an aqueous solution). The reason for this behavior arises from the limited area of the skin surface occupied by these appendages, their relatively large diffusion constants, and the nonlinear character of diffusion prior to steady state. Usually, measurements of skin permeability done in vitro are invariably measurements of steady state because it is quite difficult to detect concentrations within the first few minutes. However, this calculation was only for small hydrophilic molecules and has been shown to fail many times because it does not take into consideration the nature of the vehicle.
This theory was then extended in another report (43), where it was found that more polar steroid molecules in aqueous solutions had a longer lag time than the nonpolar ones. The reason given for this was that the diffusion constant of polar molecules is smaller and they tend to prefer the follicular route as compared with the epidermal route, and hence the long lag times and small fluxes (Fig. 2).
In still another paper (44), aqueous ibuprofen solution penetration was monitored through hydrated human skin. The parameters of the experiment obtained were fitted into the equation as given by the authors, accounting for the shunt and bulk pathway. It was found that the shunt accounted for almost 25% of the total permeation at steady state. Also, in the presence of shunt pathway, the lag time was eight minutes, which would have otherwise been 92 minutes in the absence of the appendages.
Studies Done to Investigate Physicochemical Properties of Drug and Follicular Deposition
The importance of the appendages was shown by Kao et al. (45) in mouse skin. Three different strains of mice were used: the normal-haired mice, the nonhaired mice, and the intermediate fuzzy-haired mice. The actives, benzopyrene and testos- terone, in acetone under nonoccluded conditions were then applied to the excised dorsal skin organ culture of the mice and the permeation monitored hourly by scintillation counting for 16 hours. Some of the preparations were also investigated by fluorescence microscopy. It was concluded that the hair follicles could contribute significantly to skin permeation, although there were differences in the different strains of the mice. Testosterone permeated to a higher degree than benzopyrene. It was found that for testosterone, permeation was rapid, peak absorption was observed, and peak absorption was higher and faster than in fuzzy-haired or hairless skin. Hence, it was concluded that testosterone that is absorbed extensively, the transappendageal route, played an important role in the beginning of the
experiment, but later the percutaneous absorption was important. For benzopyr- ene, that was less well-absorbed and peak absorption was not seen during the course of the experiment, indicating that the transappendageal route is the domi- nant route. It is not explained how the permeation took place for 16 hours after acetone had evaporated.
Skin permeation of two steroids, hydrocortisone (log P ¼ 1.61) and testosterone (log P ¼ 3.32), in 95% ethanol in water was evaluated in vivo on normal and artificially damaged hairless mouse skin in which sebaceous glands disappeared during healing (41). The test compounds were applied for 0.5, 2, and 6 hours. The results indicated that the permeation into the dermis and epider- mis was more through the normal skin than appendage-free skin. It was postulated that sebaceous glands probably contributed to the penetration of hydrocortisone and testosterone. Testosterone, which is a more lipophilic molecule, was found at a higher concentration than hydrocortisone in the SC. It was also observed that in the case of normal skin, the epidermal and dermal amounts of these drugs increased with application time, but this increase was smaller in the case of scar skin. It was hypothesized that the sebaceous glands acted as reservoir for the steroids. In these experiments, it was not mentioned whether they were carried out under occluded conditions and if the vehicle evaporated before the end of the experiment.
In another study done by Hueber et al. (41), percutaneous absorption of steroids on human skin was examined in vitro. Percutaneous absorption studies on four steroids—progesterone (log P ¼ 3.87), testosterone (log P ¼ 3.32), estradiol (log P ¼ 2.49), and hydrocortisone (log P ¼ 1.61)—were done using freshly excised normal and scar (obtained from abdominal and mammary plasties) human skin. The steroids were dissolved in 95% ethanol in water and applied. The experiments were carried out for eight hours for progesterone and testosterone and up to 24 hours for estradiol and hydrocortisone. It was found that permeation of the steroids was significantly greater in normal skin as compared to scar skin. The fluxes obtained for progesterone and testosterone were significantly higher in normal skin than in scar skin. The fluxes of estradiol and hydrocortisone became significant after four hours. From the percentage absorbed f[(Normal skin – scar skin)/normal skin] 100g, it was also concluded from the study that transfollicular absorption was higher for progesterone and testosterone, the more lipophilic steroids, than for estradiol and hydrocortisone. It was not mentioned if the experiment was
carried out under occluded or nonoccluded conditions.
Viprostol is a synthetic PGE2, is a vasodilator, and its deposition was monitored in the skin, following application in different animals (mouse, rat, guinea pig, rabbit, and monkey) using scintillation counting and autoradiography (46). The active was incorporated in petrolatum base. Autoradiography was done only for mice and monkeys. In mice, the radioactivity was visible in SC and the hair shafts by 30 minutes. In two hours, radioactivity was also visible throughout the viable epidermis and only the hair shaft had considerable radioactivity. Skin taken after 12 hours indicated the presence of radioactivity well down in the follicle and, after 72 hours, radioactivity was visible only in the hair shaft and hair follicle. The monkey skin also showed a similar pattern, but the time was longer. By scintil- lation counting, it was concluded that the drug was present in the skin for a rela- tively long period, following removal of the drug from the application site. The authors indicated that there was formation of a significant drug depot in the skin and that viprostol penetrated through the follicular route.
Rutherford and Black (47) used autoradiography to study the localization of germicides in guinea pig skin. The deposition of two germicides, zinc pyride-2- thione 1 N oxide [zinc pyrithione (PTO)] and zirconium pyride-2-thione 1 N oxide (zirconium PTO), from shampoos in guinea pig skin was investigated. Appli- cation on the skin was rubbed for 10 minutes, after which the animals were killed and the skin excised. Both germicides were observed in the SC, hair follicles, and sebaceous glands. However, while zirconium PTO was detected in the upper epi- dermis, zinc PTO was not. It was concluded that zinc PTO’s solubility in sebum may allow it to become localized in the hair follicle, zirconium PTO is not soluble in sebum, and this causes it to penetrate the dermis. This suggested that the physicochemical properties of the drug influence delivery into the PSU.
Studies Done to Investigate Vehicle Effects on Follicular Deposition
Vehicles Containing Organic Solvents
MacKee et al. (48) were among the first to investigate the nature of vehicle influen- cing the deposition of iron, bismuth, sulfonamide compounds, and dyes in both guinea pigs and human skin. The penetration was monitored for 5, 30, and 60 minutes. The vehicles investigated were (i) ointment bases, which included lanolin, (ii) organic solvents, which included PG, (iii) aqueous solutions, (iv) mixtures of water, surface active agents, and solubilizers, (v) mixtures of organic solvents, surface active agents, solubilizers, and coupling agents, (vi) mix- tures of organic solvents, surface active agents, and solubilizers, (vii) mixtures of organic solvents, water, surface active agents, and solubilizers, and (viii) mixtures of organic solvents, surface active agents, and water. None of the studies were above one hour. Histological examination of the skin biopsies was done to investi- gate the distribution of the “active.” There was little or no penetration seen by the ointment bases, or by PG, better penetration with the aqueous solutions with the surface-active agent and best penetration with the combination vehicles. The authors first suggested the follicular route as being important for delivery of actives. In this particular study, the time in which it was carried out was very short, and it is possible that due to the viscosity of the oleaginous bases, the effect was not visible until one hour.
Rutherford and Black (47) used autoradiography to study the localization of germicides in guinea pig skin. A trichlorocarbanilide (TCC) compound in soap vehicle resulted in predominantly transepidermal penetration through the skin, whereas follicular deposition occurred with a nonsoap detergent. Application on the skin was rubbed for 10 minutes, after which the animals were killed and the skin excised. They concluded that the vehicle influenced the penetration route.
Montagna (49) studied penetration and local effect of vitamin A on the skin of the guinea pig. The effect of vehicles, linoleic acid, oleic acid, alcohol, and chloro- form and a paste made of petrolatum, zinc oxide, and talcum on penetration of the 0.5% vitamin A active was investigated. Specimens of skin were in contact with these agents for 10 minutes, one hour, and two hours and, in some cases, four and eight hours, after which skin biopsies were taken. The penetration was tracked by the fluorescence of vitamin A. It was found that the penetration of the active (dissolved in alcohol and chloroform) into the PSU was seen very quickly (in 10 minutes). In oleic acid, the penetration to the sebaceous ducts took almost two hours. With linoleic acid, it was almost four to eight hours before fluorescence could be detected in the sebaceous ducts. With the paste, it was even slower. The
author concluded that the speed of vitamin A depended on the vehicle used. It is possible that viscosity effects played a role in delivery to the PSU.
Estradiol distribution and penetration was penetration of studies in rat skin after topical application, by high-resolution autoradiography (50). Estradiol in differ- ent concentrations was applied in vehicles dimethyl sulfoxide (DMSO), ethylene glycol, and sesame oil in vivo. It was observed that the rate of estradiol localization in the sebaceous glands was dependent on the vehicle and dose. At the end of two hours, the rate of deposition of the drug into the sebaceous glands was more with DMSO as compared to ethylene glycol. The concentration of estradiol is found to be the highest in the sebaceous glands at the end of two hours, after which it starts to decrease. It was observed that radioactivity was retained in the sebaceous glands for 24 hours or longer in low but significant amounts in all vehicles, suggesting that a drug depot effect may occur within the PSU.
The importance of the appendageal pathway was also observed in the percu- taneous absorption of 5% pyridostigmine bromide (hydrophilic) through various vehicles, which was evaluated using normal and appendage-free scar rat skin in vitro during 72 hours (51). It was found that the drug absorbed was higher from nerol 8% in ethanol, followed by azone 5% in ethanol – PG (90:10), followed by DMSO 10% in ethanol. PG 10% in ethanol inhibited pyridostigmine absorption as compared to the control, which was an ethanolic solution. In all cases, the absorp- tion through the appendage-free skin was lower than the absorption through control skin. The percentage of appendageal pathway was calculated by the formula (1-Scar skin flux/normal skin flux) 100. It was found that in the first four hours, the appendageal absorption was most for the DMSO solution, followed by the proplylene glycol in ethanol, followed by ethanol, then azone solution, and then the nerol solution. At the end of 72 hours, proplylene glycol in ethanol had the highest percentage of appendageal transport, followed by DMSO, and the others were not that significant. The authors suggested that the enhancers (Nerol, azone) affected the structure of the epidermis, whereas the other solvents (DMSO, ethanol, and PG) were incorporated in the sebum and dragged the drug into the sebaceous ducts. These experiments were done under occlusion. It was con- cluded that ethanol, DMSO, and PG in ethanol favored the transfollicular pathway, but the other vehicles did not. Since ethanol is primarily a lipid solvent, it can solu- bilize sebum and allow the migration of the active in the sebaceous glands, explain- ing why it is primarily transfollicular. It was concluded that by using the right vehicle it was possible to favor the transfollicular pathway and target the drug.
Radiography was used quantitatively by Fabin et al. (39). Two drugs, tetra- hydrocannabinol (THC) and oleic acid, were evaluated for delivery into hairless rat skin appendages with different vehicles in vivo. These vehicles included poly- ethylene glycol 400 (PEG 400), transcutol, and PG:ethanol (7:3). It was found that after two hours, THC had the highest penetration from transcutol and the lowest from PEG 400. After two hours, distribution of THC and oleic acid from transcutol was not very different. At 24 hours, the transcutol system had delivered the maximum THC in the different skin layers and PEG 400 delivered the lowest. At
24 hours more, THC had been delivered in the different layers of the skin compared two hours. It appears that there is a time-dependent effect in the distribution and localization of the drug in the follicle. In the same experiments, when oleic acid was added as a vehicle to the PG:ethanol system, the penetration of THC after two hours was much higher with compared to when oleic acid was not added. It was concluded that the presence of oleic acid in the delivery system applied to
the skin could increase penetration to all the strata of the skin, including the appendages.
The percutaneous absorption of RA was monitored in haired and hairless guinea pigs with the expectation that drug penetration would be lower in hairless guinea pigs because they have fewer hair follicles (30). RA was formulated in a
0.025% in an ethanolic gel formulation together with and without polyolprepoly- mer (PP-2) at 10%. In vitro permeation was monitored on haired and hairless guinea pigs for 24 hours. It was observed from penetration profile that hairless guinea pig skin was more permeable to RA than haired skin despite the lower follicular density. This was attributed to the different strains having different thickness of the SC and different structure rather than greater follicular density. It was suggested that a depot of RA was formed in the hair follicle of the hair guinea pig due to the PP-2. The addition of PP-2 to the formulation decreased the penetration of the drug. It is possible that by addition of a polymer, the viscosity of the formulation increased and hence there was decrease in the penetration.
In summary, from these studies it can be concluded that vehicles that interact with sebum seem to be delivering the drug into the PSU. If sufficient time is given for the experiment to run, then the more viscous materials may help in delivering the drug. Viscosity of the vehicle may play a role in the delivery.
Vehicles Made of Liposomes
Liposomes have become popular in the delivery of drugs to the PSU. In a study done by Li et al. (52), liposomes made of phosphatidylcholine (PC)-containing calcein dye were investigated for delivery to the PSU using mouse skin histocultures by confocal microscopy. It was found that liposomes-entrapped dye became associated with the hair follicles in contrast to free dye without liposomes in 20 minutes. In a similar study (53) done by the same authors, liposome, made of PC, mediated targeted delivery of melanin and into the hair follicles of histocul- tured mouse skin in 12 hours was reported. This was monitored by fluorescence microscopy. Similarly, DNA encapsulated liposomes were shown to target the hair follicle of histocultured mouse skin in 44 hours, as shown by autoradiography (54). For all these studies, the entrapped drug was separated from the free drug using gel filtration. It could be concluded that it takes longer for larger mol- ecules to be targeted to the PSU.
Liposomes have been used by Lieb et al. (18) for delivery into the PSU of the hamster ear in vitro. Carboxyfluorescein (CF) encapsulated in multilamellar ves- icles was prepared. The multilamellar vesicles were made of PC:Cholesterol (CH):Phosphatidylserine (PS) in the ratio of 1:0.5:0.1, respectively. Other vehicles containing the same concentration of CF were also used, which included HEPES buffer (pH ¼ 7.4), 5% PG in HEPES, 10% ethanol in HEPES, and 0.05% sodium lauryl sulfate in HEPES. In vitro diffusion studies for 24 hours, quantitative fluorescence microscopy, and scraping technique was done to investigate the depo- sition into the follicles. It was found that the most intense fluorescence was observed with the liposomal formulations, whereas the other formulations were not much different from each other.
Liposomes made of PC:CH and PS at a mole ratio of 1.0:0.5:0.1 were made containing 0.5% cimetidine (55) and were evaluated for deposition into the PSU of hamster ears. Nonionic liposomes made of GDL:CH:POE-10 at a weight ratio of 57:15:28 containing cimetidine were also evaluated in vitro and in vivo. These formulations were compared with (i) aqueous solution of pH ¼ 8.3, (ii) 50%
alcohol solution of pH ¼ 7.4, (iii) aqueous solution of pH ¼ 5.5, (iv) phospholipid liposome pH ¼ 5.5, (v) phospholipid liposome pH ¼ 8.3, (vi) nonionic liposome of pH ¼ 5.5, and (vii) nonionic liposome of pH ¼ 5.5. At pH ¼ 8.3, the drug is pre- dominately unionized. In vitro deposition studies were done on excised hamster ears when the formulation was applied for 24 hours, after which the cells were dis- mantled and the distribution of the drug in the various strata of the skin was deter- mined. In the in vitro studies, the aqueous solution showed significant deposition into the sebum-rich PSU of the hamster ear. In the in vivo studies done similarly, after 12 hours, maximum deposition in the PSU was observed by the phospholipid liposome at pH ¼ 5.5 as compared to the 50% alcohol solution at pH ¼ 7.4 and the nonionic liposome at pH ¼ 5.5. Therefore, when bioassay was done and a decrease in size of the sebaceous glands was monitored, it was found that only the hydroal- coholic solution and the nonionic liposomes suppressed the growth of the glands. From these studies it was found that there were discrepancies in the in vivo and the in vitro data, and caution needs to be exercised about the activity of the drug in specific tissue.
Influence of nonionic liposomal composition on topical delivery of alpha interferon (a-IFN) into PSUs was done using the hamster ear model (56). The depo- sition of hydrophilic protein, a-IFN into the PSU and other strata of the hamster ear 12 hours after topical in vivo application from three nonionic formulations, a phospholipid formulation, and an aqueous control was determined. The depo- sition of cyclosporin (CsA), a hydrophobic peptide into the PSUs and other strata of the ear using the same liposomal formulations and a hydroalcoholic control, was also done. The nonionic liposome (Non-1) was made with GDL, POE-10, and CH, Non-2 was made with GDS (glyceryl distearate) and POE-10 and CH, and Non-3 was made with POE-10 and CH. The liposomal formulation was made from PC:CH and PS at a mole ratio of 1.0:0.5:0.1. The analysis was done the scintillation counting. The deposition a-IFN of into the PSUs was in the order: Non-1 .. PC . Non-2 . Non-3 ¼ Aq. The deposition of CsA into the PSUs was in the order: Non-1 .. Hydroalcoholic solution . PC . Non-2 ¼ Non-3. It was concluded that in spite of differences in the hydrophobicities of the peptide drugs, Non-1 liposome significantly enhanced the deposition into the PSUs. Hence, the vehicle carrying the drug made a major impact on the deposition irrespective of whether it was hydrophilic or hydrophobic. CsA was, however, deposited to a greater extent than a-IFN. It was explained that since GDL melted at 308C, it was able to cause some fluidization of the liposomal bilayer and partial release of its contents like POE-10 at body temperature, whereas GDS melted at 548C and hence did not cause this fluidization.
Expression plasmid DNA for the human interleukin-1 receptor antagonist (IL-1ra) protein was formulated with nonionic:cationic (NC) liposome and PC:cationic (PC) liposomes and applied to auricular skin of hamsters (57). Confocal microscopy identified delivery of the plasmid DNA proximal to the perifollicular cells. In the second part of the study, the skin was treated for three days with the NC liposomes and had statistically significant levels of transgenic IL-1ra present for five days post-treatment. The results indicated that the NC liposomes could deliver expression plasmid DNA to perifollicular cells and mediate transient trans- fection in vivo. The control formulations were made with empty niosomes, that is, no DNA, and were done with infinite dosing. The control used should have been empty niosomes with the plasmid DNA outside it and would have indicated if drug or niosome had the property of reaching the follicle.
The sebaceous gland deposition of isotretinoin after topical application on human facial skin in vitro was measured 16 hours after application by taking skin biopsies and separation of the skin compartments (58). Ethanolic gel was the control and was compared with a liposomal formulation, Natipide II (made of PC) formulation, and a mixed micelle formulation. All the experiments were per- formed with skin from the periauricular skin of women undergoing plastic surgery. Autoradiography and fluorescence microscopy elucidated the penetration pathway of the active. The results showed that neither the liposomes nor the mixed micellar system revealed an improved sebaceous gland deposition of isotretenoin as compared with the ethanolic gel.
The significance of the sebaceous gland pathway in the cutaneous permeation of an antiandrogen, RU58841 in liposomes, was studied with normal and scar skin in hairless rat (59). RU 58841 was made into a liposomal formulation containing lipoid E 100-35 and a-tocopherol in a phosphate buffer of pH ¼ 7, whereas the control solution was made of ethanol PG:water (40:10:50 w/w). The in vitro cutaneous permeation studies were carried out for 24 hours. The cumulative per- centage of RU 58841 absorbed was three-fold higher through normal skin after 24 hours, whereas that of the liposomes was four-fold higher through normal skin as compared to scar skin. However, the permeation of the ethanolic solution was much higher through normal skin as compared to the liposomal formulation through the skin. In in vivo studies carried out for 24 hours, it was found that the epidermis and dermis of normal skin contained more amounts of the active than the scar tissue. An autoradiography showed that with the ethanolic sol- ution, the drug was mainly localized in the SC/epidermis and with the liposomes it was localized in the sebaceous glands. The overall permeation was, however, higher through the normal skin for the ethanolic solutions as compared to the lipo- somal solutions. When the correction for targeting was applied, liposomes targeted better than the ethanolic solution.
Mechanism of Action of Liposomes
Weiner et al. (60) have hypothesized the use of liposomes in the following way. For hydrophobic drugs, a major fraction of the added drug would be encapsulated or intercalated within the bilayers of the liposomes. The transfer of drug from the lipid bilayers into the skin could occur as long as the bilayers were in liquid crystal- line state. If the liquid crystalline phase is altered to the gel state, transport of the drug would cease or be negligibly low. Dehydration of liposomal suspensions has been shown to reduce transitions from the liquid crystalline phase to the gel state. If, however, dehydration was complete and the bilayers were transformed from the liquid crystalline state to a gel state, then the transfer of the drug would cease. If dehydration to an equilibrium stage occurs, wherein a constant amount of water is always retained, then the transport of the drug is steady and continuous. A second consequence of dehydration involves the formation of a strong adhesive patch of the liposomal bilayers on the skin. The formation of such patches maxi- mizes the intimacy of contact between the drug-laden bilayers and the skin.
For hydrophilic drugs, too, a similar mode of action is explained. Liposomal systems that undergo total dehydration, drug transport ceases, as the drug is no longer in a dissolved state. For liposomal systems that retain a constant amount of water within the bilayers following dehydration to an equilibrium, drug trans- port would continue over extended periods. A major consequence of dehydration for hydrophilic drugs involves the enhancement or enrichment of drug
concentration in the aqueous phase of the bilayers, leading to an enhancement in flux of drug into and across skin. When a follicular pathway is present, upon dehy- dration, the liposomal bilayers can partition and can pack into the follicular ducts containing the lipids. The filling of the follicular openings with the liposomal bilayers not only results in entrapped drugs being carried into the follicles, but also allows partitioning of free drugs into the bilayer matrix within the follicles.
Vehicles Made of Lipid Melts
In these studies, the melted form of the lipid components, which were used to make the niosomes (namely, GDL:CH:POE-10), were evaluated as a vehicle for the trans- port of CsA and other agents into and through hairless mouse skin (58,60,61). The experiments were carried out under nonoccluded conditions. At predetermined times of two, four, eight and 24 hours, the diffusion setup was dismantled and the drug in various strata of the skin determined using stripping and digesting of the skin using scintillation counting. The profiles of the extent and uptake of CsA from the lipid melt formulations are similar to those of the liposomal formulations of the same compositions. In these same experiments, the ratio of GDL to POE-10 was varied. In all these formulation, CH was kept at a constant concentration of
15% and GDL to POE-10 ratios examined were 0:85, 15:70, 45:40, and 57:28. It was found that the 45:40 lipid melt was more effective in delivery of the active into and through the mouse skin. The authors explained that that particular combination of lipid melt had the lowest melting point ( 238C) and that was responsible for the better uptake. Nevertheless, the remaining melts (except 0% GDL) were also liquids at body temperature and the melting point does not seem to explain the point enough. The microautoradiographs and light microscope images also confirm these results. It was thus concluded that the similarity of the kinetic profiles for CsA transport into and across living skin strata of hairless mice from the lipo- somal and lipid melt formulations along with the microautoradiographic evidence for localization within the PSUs implied that transport of CsA into the highly hydro- philic viable skin strata occurs mainly via the follicular route.
Similar studies were also done on the delivery of hydrocortisone (hydro- phobic) and mannitol (hydrophilic) through the same lipid-based formulations in and through hairless mouse skin in vitro (62). The results after 24 hours indicated that the extent of hydrocortisone uptake rose with increasing the GDL to POE-10 ratio, whereas for mannitol, the uptake was the opposite decreasing with increase in this ratio in liposomes. Lipid melt formulations for mannitol were not made because it is a hydrophilic drug and niosomes of the above compositions along with water were made. From the autoradiographic studies, it was reported that the nonionic lipid-based formulations were predominantly transfollicular. It was also suggested that a hydrophilic molecule like mannitol could not partition into the lipid environment of the sebum-filled follicles or into and across the SC and was transported mainly via the transepidermal route. This suggested that the two macromolecules were transported through the skin via two different pathways. It was thus was possible to tailor formulations for specific and targeted delivery across a certain route.
Studies Done on Microbeads: Size Effects on Follicular Deposition
Fluorescent polystyrene (16) microbeads of different sizes were made in aqueous medium and Miglyolw (hydrophobic vehicle) as suspensions and were applied on live female hairless rat’s back and freshly excised human skin. Suspensions were applied with massaging for five minutes. The experiment was carried on
for 15 minutes, after which skin biopsy was done and observed under a fluorescent microscope. It was found that the best follicular penetration was observed with the most lipophilic vehicle Miglyolw. The 9- to 10-mm beads concentrated at the opening of the follicles, but did not penetrate; however, the 7-mm beads were fre- quently observed deep down in the follicular canal, but rarely penetrated the SC. The smallest beads ,3 mm penetrated the follicles well, but were also observed in the superficial layers of the SC.
In the same paper, poly b-alanine beads labeled with dansyl chloride were formulated in an aqueous gel, hydroalcoholic gel, and silicone oil. The suspensions were applied to the rat’s back and human skin. Here too, the best results were observed with the most lipophilic vehicle (silicone oil). The 5 mm showed selectivity for the follicular canal.
The site-specific delivery of adalapene to the hair follicles was attempted both in vivo and in vitro (63). The drug was loaded in 50:50 poly (DL-lactic – coglycolic acid). Different sizes (1, 5, and 20 mm) of the microspheres were evaluated in vitro by investigating cutaneous penetration of the microspheres through human skin and the female hairless rat skin. The permeation was allowed to proceed for
35 and 300 minutes, after which the cells were dismantled and the skin samples were frozen and then evaluated by fluorescence microscopy. It was found that the 1-mm microspheres were randomly distributed into the SC and hair follicles. The 20-mm microparticles did not penetrate the skin and remained on the surface where the 5 mm were found in the hair follicles. In addition, a time-dependent effect was seen in this evaluation where greater penetration was observed after
300 minutes than after 35 minutes.