Boris Giannaccini

Evaluation of muco-adhesive properties and in vivo activity of ophthalmic vehicles based on hyaluronic acid

Abstract A series of prospective ophthalmic vehicles based on hyaluronic acid (HA) and on polyacrylic acid (PAA) (solutions, gels, matrices prepared by compression and by casting) containing pilocarpine (Pi) or tropicamide (Tr) was evaluated for muco-adhesion, for ocular retention and for biological activity (miosis, mydriasis) in rabbits. The muco-adhesive properties were investigated in vitro using a tensile apparatus with mucin-coated surfaces, while the ocular behaviour was estimated visually, using vehicles containing a fluorescent marker. Good to excellent muco-adhesive properties were detected in the HA preparations. The bioavailability-enhancing effect, however, was not very satisfactory with Pi, probably on account of the high solubility and diffusivity of the drug. The effect was more evident with the less soluble drug Tr. The validity of the method used for evaluating bioadhesion, and the relevance of the physicochemical characteristics of the drug to a muco-adhesive ocular delivery system are discussed.

Vehicle effects on ophthalmic bioavailability: the influence of different polymers on the activity of pilocarpine in rabbit and man

Viscosity-increasing polymers are usually added to ophthalmic drug solutions, on the premise that an increased vehicle viscosity should correspond a slower elimination from the preocular area, and hence a greater transcorneal penetration of the drug into the anterior chamber. Many polymers have been screened in the attempt to determine the ideal polymer and viscosity, and to establish quantitative relationships between vehicle viscosity, retention time and ophthalmic bioavailability. The experiments are usually carried out by the different authors on rabbits or man, and this has generated some confusing and contradictory data, since the two species show important differences in ocular anatomy and physiology (e.g., different rates of blinking, tear secretion and turnover, drainage of instilled fluid, etc) that may produce different responses to vehicle viscosity. In spite of some diverging reports as the effect, in man and in rabbits, of the viscosity of the vehicle on bioavailability, there appears to exist an implicit agreement on the thesis that, within each species, vehicles prepared with different polymers should behave identically when compared on an equal viscosity basis (Patton & Robinson 1975). In the present study, three Newtonian equiviscous vehicles and one pseudoplastic vehicle, all prepared with different polymers and containing pilocarpine were tested on rabbits and man. The aims of the investigation were (a) to verify the equal viscosity-equal activity assumption, i.e., the alleged lack of influence on activity of the type of polymer; (b) to define species differences in the biological response to the same vehicles; and (c) to assess the relevance to activity of the type of flow of the vehicle (Newtonian vs pseudoplastic). All preparations tested were made by adding the appropriate amount of polymer to an aqueous isotonic buffered (Sarensen 0.2 M phosphate buffer, pH 5 .5 )

Polymer effects on ocular bioavailability: the influence of different liquid vehicles on the mydriatic response of tropicamide in humans and in rabbits

Five viscous polymeric vehicles containing 0.2% tropicamide were tested for mydriatic activity in humans and in rabbits. The polymers were carboxymethylcellulose (CMC), low molecular weight hydroxypropylcellulose (HPCL). medium molecular weight hydroxypropylcellulose (HPCM). poly(vinylpvrrolidone) (PVP) and poly(vinyl alcohol) (PVA), Their concentrations were adjusted in order to give iso-viscous (70 ± 2 cps) Newtonian solutions (HPCL, PVP, PVA) or pseudoplastic solutions with an apparent viscosity of 70 cps at a rate of shear of 700 s−1 (CMC. HPCM). All vehicles increased the ocular bioavailability of the drug in both species, when compared with a non-viscous solution. However, in humans PVP and PVA were significantly more active with respect to the other polymers. The different activity of the polymeric vehicles in humans could neither be correlated with drug binding, nor with surface or interfacial tension, nor with the rheological behaviour of the solutions (Newtonian vs pseudoplastic). On the basis of experimental evidence, the hypothesis is advanced that some polymers may increase the ocular bioavailability of tropicamide not by viscous effects aione, but also by influencing the spreading characteristics and the thickness of the medication layer over the precorneal area. The reasons for the interspecies differences in activity, presumably residing in different precorneal dynamics of the applied solution, are also discussed.

Evaluation of high- and low-molecular weight fractions of sodium hyaluronate and an ionic complex as adjuvants for topical ophthalmic vehicles containing pilocarpine

Ophthalmic vehicle; Hyaluronic acid; Pilocarpine; Miotic test; Ocular permanence test; Rabbit; Bioadhesion Summary Two low-molecular-weight fractions of sodium hyaluronate (Na-HA), denominated Hyalastin® and Hyalectin®, were investigated as potential adjuvants for ophthalmic vehicles containing pilocarpine nitrate (PiN). Tests were also performed on an ionic complex (HA/PiB) prepared from hyaluronic acid (derived from Hyalastin®) and pilocarpine base. The performance of the vehicles under study was verified by miosis and ocular retention tests carried out on albino rabbits, against a series of reference vehicles, three of which contained a high-molecular-weight fraction of Na-HA (Healon®). The group of 14 reference and test preparations exhibited Newtonian or pseudoplastic flow characteristics and encompassed a wide range of apparent viscosities (1 to 1054 mPa s). The results indicate that the HA/PiB salt and the high-MW Na-HA can significantly increase the bioavailability of pilocarpine with respect to reference vehicles of comparable viscosity: an effect that can be reasonably attributed to muco-adhesive effects. Conversely, in the present rabbit tests, the low-MW fractions of Na-HA performed poorly as adjuvants for the PiN solutions.

Solubilization of tropicamide by poloxamers: physicochemical data and activity data in rabbits and humans☆

Abstract A series of polyoxyethylene-polyoxypropylene (POE/POP) block copolymers (Poloxamers, or Pluronics) were evaluated as solubilizers for tropicamide, a poorly water-soluble mydriatic/cycloplegic drug. The selected Pluronics were L64, P65 and F68 (POP weight 1750, POE weight 1150, 1650 and 6650, respectively), P75 and F77 (POP weight 2050, POE weight 2100 and 4550, respectively), P84, P85, F87 and F88 (POP weight 2250, POE weight 1950, 2350, 5450 and 9150, respectively) and F127 (POP weight 4000, POE weight 8600). The following studies were carried out: solubility of tropicamide in polymer solutions, partition coefficient of the drug between isopropyl myristate and polymer solutions, critical micelle concentration (CMC) of the polymers, viscosity of the polymeric solutions containing tropicamide, mydriatic activity tests on rabbits and humans, cycloplegic activity tests on humans. The solubility isotherms (25 °C) showed that the saturation solubility of the drug increased linearly with increasing surfactants concentration in the 4.0–20.0 w/v concentration range. In the presence of 20% w/v Pluronics the drug solubility increased substantially, ranging from 1.9 times (F88) to ca. 3.0 times (P85) the solubility in water at the same temperature (0.57 g/100 ml). Analysis of the solubility data indicated that the solubility of tropicamide increased as the oxyethylene content of the surfactants increased, and that the amount of drug solubilized per EO unit decreased with increasing hydrophilicity (increasing OE chain length) of surfactants. Calculation of the relative amount of drug bound to the POE and to the POP portions of the surfactant molecules indicated that binding occurs in part to the hydrophilic (POE) outer mantle, and in part to the hydrophobic, (POP) inner core of the micellar aggregates, with POE/POP binding ratios varying from 1.17 to 3.13, depending on the polymer type. Biological activity tests were carried out with some 15% w/v polymeric solutions (L64, P75, P84, P85 and F87) containing 1.0% w/v tropicamide, and with some 20.0% w/v solutions (P85, F87) containing 1 5 % w/v drug. The results indicate that tropicamide bioavailability, both in rabbits and in humans, was not decreased by micellar Solubilization, and that some Poloxamers can perform satisfactorily as solubilizing vehicles for tropicamide, producing neutral 1.0% and 1.5% drug solutions which are better tolerated and more effective than the standard aqueous eyedrops.

Vehicle effects in ophthalmic bioavailability: an evaluation of polymeric inserts containing pilocarpine

A series of polymeric ophthalmic inserts containing pilocarpine were formulated with four different types of polyvinyl alcohol, PVA, and two types of hydroxypropylcellulose. Pilocarpine was present as the nitrate, or as the salt with polyacrylic acid, PAA. In‐vivo miosis vs time experiments on albino rabbits, showed that all inserts increased significantly the bioavailability of pilocarpine, with respect to a standard solution of pilocarpine nitrate. Two PVA inserts, containing the PAA‐salt of pilocarpine, were particularly effective. The preparations were also submitted to in‐vitro release tests and to differential scanning calorimetry, to ascertain the release mechanism, and to verify, via the thermal behaviour, possible interactions between drug and polymers. The chemical and physicochemical factors, most likely to influence the ophthalmic bioavailability of pilocarpine from the present preparations, are briefly reviewed.

Semisolid ophthalmic vehicles. III: An evaluation of four organic hydrogels containing pilocarpine

Abstract The validity of four hydrogels, based on synthetic polymers as vehicles for topical ophthalmic drugs was evaluated by determining, in rabbits and in humans, the miotic response induced by pilocarpine. The hydrogels were prepared with two different types of poly(acrylic acid), PAA-1 and PAA-2; with poly(acrylamide) PAAm and with ethylene maleic anhydride, EMA. Their biological effect was compared with that induced by an aqueous solution of the drug (AS). The rheological characteristics of the vehicles were investigated: PAA-1, PAA-2 and EMA displayed a plastic type of flow, while PAAm was pseudo-plastic. In rabbits administration of pilocarpine in the hydrogel vehicles doubled the drug bioavailability (as expressed by the area under the miotic response vs time curve, AUC) with respect to AS; however, no statistical differences were apparent among the AUC values of the hydrogels. In humans, the same vehicles showed more pronounced activity differences. While PAA-2 and PAAm produced an approximately 3-fold bioavailability increase with respect to AS, PAA-1 only doubled the bioavailability and EMA showed activity parameters not different from those of the aqueous solution. Possible correlations between the viscosity parameters of the vehicles and ophthalmic bioavailability in the two species are discussed. The present results appear to confirm previous observations on the poor validity of rabbits for studies on the influence of vehicle viscosity on bioavailability of topical ophthalmic drugs.

Enhancement of transdermal penetration of dapiprazole through hairless mouse skin.

The in vitro permeation rate of dapiprazole base (DAP-B) through hairless mouse skin was investigated, as a preliminary step towards the development of a transdermal therapeutic system. The study involved the evaluation of the permeability coefficient of the drug applied to the skin in a series of liquid and semisolid vehicles, both in the absence and in the presence of different penetration enhancers. In liquid vehicles the permeability coefficient of DAP-B was significantly promoted (up to 73 times) by some terpenes (1-limonene, α-bisabolol, terpinolene) and by a mixture of unsaturated fatty acids. Similar effects were noted in semisolid vehicles, although the permeability coefficients were lower. Iontophoretic experiments on DAP-B in physiological saline solution, at constant current densities in the range of 0.05-0.5 mA/cm2, produced up to 115-fold permeability increases relative to passive diffusion. The present results, even if needing further corroboration by tests on human skin, evidenced the activity of some molecules as skin permeation enhancers for DAP-B, and confirmed the synergy between propylene glycol and the enhancers, already reported in the literature. The possibility of promoting DAP-B transport through the skin by iontophoresis was also established.

The effect of different ophthalmic vehicles on the activity of tropicamide in man.

The most common vehicles for topical ophthalmic drugs are ‘regular’ saline solutions, viscous saline solutions and semisolid paraffin ointments. Semisolid aqueous gels are seldom used clinically, in spite of evidence of greatly enhanced therapeutic responses in the rabbit (Giroux & Schrenzell964; Bottari et a1 1979). Whether viscous saline solutions are superior to ‘regular’ ones in increasing the drug bioavailability in man has been occasionally raised (Linn & Jones 1968; Mattila et al 1968; Waltman & Patrowicz 1970; Adler et a1 1971; Trueblood et al 1975; Melis-Decerf & Van Ooteghem 1979) and is still apparently unsettled. The present study was undertaken to evaluate critically on human subjects (a) the effect of a viscous vs a ‘regular’ solution, and (b) the effect of a n aqueous gel vs a viscous solution and an ointment. Vehicles containing three increasing concentrations (0.05, 0.1 and 0.2 % w/v) of tropicamide were used. Commercial tropicamide (Prodotti Roche) was purified to constant m.p. 97-99°C. A sample was micronized (Fryma JMRS-80 jet mill) to yield particles of average diameter (geom., microscopic analysis) 4.5 pm. The ‘regular’ aqueous saline solutions (AS) were prepared with 0.2 M, pH 7.0 Ssrensen phosphate buffer, whose tonicity was adjusted with NaCI. The same vehicle, containing 0.7 % w/v hydroxypropylcellulose (Alcogel, Lab. Vevy) was used for the preparation of the viscous solutions (VS). The aqueous gels (AG) were prepared by neutralizing with diisopropanolamine (Fluka A.G.) 0.3 % w/v dispersions of carboxyvinyl polymer (Carbopol 940, B.F. Goodrich Chemical Co.) containing the appropriate amount of tropicamide. All of these preparations were autoclaved at 2 bars for 20 min. The pH after sterilization was in the range 64-7.1. The ointment vehicle (OV) was a 70: 30 w/w mixture of yellow soft paraffin and liquid paraffin (both B.P. grade, Carlo Erba S.p.A.). Micronized tropicamide was incorporated into this vehicle by levigation under sterile conditions : no final sterilization was carried out. The preparations were tested on 210 Caucasian volunteers of either sex, aged 15-65 years: subjects with conjunctival or corneal abrasions and disorders, or glaucoma, were excluded. Each concentration was tested on groups of at least 20 subjects. The horizontal diameter of the pupil was estimated to the nearest 0.1 mm with a micrometer held always at the same distance from the subjects’ face, by the same operator.

Polymer effects on ocular bioavailability. II. The influence of benzalkonium chloride on the mydriatic response of tropicamide in different polymeric vehicles

The effect of 0.01% benzalkonium chloride (BZ) on the mydriatic response of tropicamide in different vehicles was tested on rabbits and on humans. The vehicles were an isotonic buffered solution (AS) and four iso-viscous polymeric solutions: low molecular weight hydroxypropylcellulose (HPCL), medium molecular weight hydroxypropylcellulose (HPCM), poly(vinyl alcohol) (PVA) and poly(vinylpyrrolidone) (PVP). In rabbits, the BZ-containing vehicles did not show any statistically significant activity difference respect to identical, BZ-free preparations. In humans, BZ did not produce any significant bioavailability increase in AS, even at the 0.05% concentration, while it appeared to depress the bioavailability enhancement produced by the polymeric solutions. This effect was statistically significant (P < 0.05) for PVA and PVP. On the basis of some physicochemical evidence, the hypothesis is advanced that the reduced efficacy in humans of the polymeric vehicles in the presence of BZ may be due to a reduced adhesion tension (AT) of the solutions to the corneal surface. This would interfere with the blinking-assisted spreading process of the vehicles over the precorneal area. The present experiments confirmed the poor performance of rabbits as models for investigations on polymeric ophthalmic vehicles, that may modulate the drug availability not by viscous effects alone, but also by dynamic surface spreading phenomena.