Relationship between the ocular and systemic disposition of flurbiprofen: The effect of altered protein dynamics at steady state

The differences in flurbiprofen disposition in the aqueous humor and the plasma were examined after systemic doses. Steady state plasma concentrations of flurbiprofen (20–60 g/mL) were achieved via intravenous infusion to albino rabbits. Flurbiprofen demonstrated linear systemic kinetics throughout the dosing range, with constant body clearance and unbound fraction in plasma. At steady state, aqueous humor drug concentrations depended on the corresponding plasma drug concentration. Two clearance terms—CL_so, the systemic clearance to ocular tissues, and CL_os, the ocular clearance to systemic circulation—were used. After systemic doses, the drug concentration in the aqueous humor was related to that in the plasma as well as to the ratio of these two clearances. Flurbiprofen was extensively bound to plasma proteins and showed limited ocular distribution; its CL_so to CL_so tratio was very small. Thus, the concentration of flurbiprofen in the aqueous humor after systemic doses was lower than that obtained after ophthalmic doses. A plasmapheresis technique was utilized to lower the plasma protein concentrations to 60% of normal levels. As a consequence, flurbiprofen demonstrated reduced aqueous humor protein concentrations, increased unbound fractions in the plasma and the aqueous humor, elevated aqueous humor drug concentrations, and elevated total body clearance. The unbound body clearance stayed unchanged. Our study indicated that a drug should present a significant CL_so/CL_os ratio in order to achieve therapeutic concentrations in the eye via systemic doses. The drug-protein binding kinetics can be different between the plasma and the aqueous humor circulations. Because the ocular compariment is very small compared to the overall systemic distribution of flurbiprofen, it has little effect on the steady state systemic concentrations.     

A phase 1 study of tazarotene in adults with advanced cancer

Tazarotene is an acetylenic retinoid which is metabolised to tazarotenic acid and which binds selectively to the retinoid receptors RARbeta and RARgamma. The safety, toxicity and pharmacokinetics of oral tazarotene were determined over 12 weeks of treatment in 34 patients with advanced cancer. Commonly seen toxicities were mucocutaneous symptoms, musculoskeletal pain and headache. Dose-limiting toxicities were hypercalcaemia, hypertriglyceridaemia and musculoskeletal pain. The maximum tolerated dose of tazarotene in this schedule is 25.2 mg day(-1). Plasma concentrations of tazarotenic acid were found to peak rapidly within 1-3 h of dosing and thereafter declined quickly. The C(max) and AUC values on day 0, and weeks 2 and 4 were similar indicating no drug accumulation. The dose-normalised C(max) and AUC values at different dose levels and different study days appeared to be similar indicating linear pharmacokinetics. No objective responses were seen, although stable disease was seen in six out of eight evaluable patients receiving the three highest dose levels of tazarotene (16.8, 25.2 or 33.4 mg day(-1)). We conclude that oral tazarotene is well tolerated when administered daily for 12 weeks, has a favourable toxicity profile compared with other retinoids and merits further investigation as an anticancer therapy.     

Blood concentrations of cyclosporin a during long-term treatment with cyclosporin a ophthalmic emulsions in patients with moderate to severe dry eye disease.

To quantify blood cyclosporin A (CsA) concentrations during treatment with CsA topical ophthalmic emulsions, blood was collected from 128 patients enrolled in a Phase 3, multicenter, double-masked, randomized, parallel-group study of CsA eyedrops for treatment of moderate to severe dry eye disease. Patients received 0.05% CsA, 0.1% CsA, or vehicle b.i.d. for 6 months; vehicle-treated patients then crossed over to 0.1% CsA b.i.d. for 6 months. CsA concentrations were measured using a validated LC/MS-MS assay (quantitation limit = 0.1 ng/mL). No patient receiving 0.05% CsA had any quantifiable CsA in the blood (n = 96 samples). All but 7 of 128 (5.5%) trough blood samples from the 0.1% CsA group were below the quantitation limit for CsA; none exceeded 0.3 ng/mL. CsA was also below the limit of quantitation in 205 of 208 (98.6%) of serial postdose blood samples collected from 26 patients during 1 dosing interval between months 9 and 12. The highest C(max) measured, 0.105 ng/mL at 3 hours postdose, occurred in a 0.1% CsA-treated patient. These results indicate that long-term use of topical CsA ophthalmic emulsions at doses that are clinically efficacious for treating dry eye will not cause any system-wide effects.     

Pharmacological characterization of a novel antiglaucoma agent,Bimatoprost (AGN 192024)

Replacement of the carboxylic acid group of prostaglandin (PG) F(2alpha) with a nonacidic moiety, such as hydroxyl, methoxy, or amido, results in compounds with unique pharmacology. Bimatoprost (AGN 192024) is also a pharmacologically novel PGF(2alpha) analog, where the carboxylic acid is replaced by a neutral ethylamide substituent. Bimatoprost potently contracted the feline lung parenchymal preparation (EC(50) value of 35-55 nM) but exhibited no meaningful activity in a variety of PG-sensitive tissue and cell preparations. Its activity seemed unrelated to FP receptor stimulation according to the following evidence. 1) Bimatoprost exhibited no meaningful activity in tissues and cells containing functional FP receptors. 2) Bimatoprost activity in the cat lung parenchyma is not species-specific because its potent activity in this preparation could not be reproduced in cells stably expressing the feline FP receptor. 3) Radioligand binding studies using feline and human recombinant FP receptors exhibited minimal competition versus [(3)H]17-phenyl PGF(2a) for Bimatoprost. 4) Bimatoprost pretreatment did not attenuate PGF(2alpha)-induced Ca(2+) signals in Swiss 3T3 cells. 5) Regional differences were apparent for Bimatoprost but not FP agonist effects in the cat lung. Bimatoprost reduced intraocular pressure in ocular normotensive and hypertensive monkeys over a 0.001 to 0.1% dose range. A single-dose and multiple-dose ocular distribution/metabolism studies using [(3)H]Bimatoprost (0.1%) were performed. Within the globe, bimatoprost concentrations were 10- to 100-fold higher in anterior segment tissues compared with the aqueous humor. Bimatoprost was overwhelmingly the predominant molecular species identified at all time points in ocular tissues, indicating that the intact molecule reduces intraocular pressure.     

Comparison of tear sampling techniques for pharmacokinetics analysis: ofloxacin concentrations in rabbit tears after sampling with schirmer tear strips, capillary tubes, or surgical sponges.

This study compared the precision and accuracy of 4 tear sampling methods. In vivo, albino rabbits were treated with single bilateral eye drops ofofloxacin 0.3% solution, 3 hr after which tear samples were collected using capillary tubes (CT), surgical sponges (SS), or tear strips for 15 sec (15sTS) or 60 sec (60sTS). In vitro, CT, SS, and tear strips were spiked with known volumes of ofloxacin solution in order to assess the bioanalytical accuracy of each technique. Ofloxacin levels were quantified by HPLC in all samples. Results showed that tear volumes and ofloxacin masses sampled in vivo depended on sampling method. Tear volume followed the rank order 60sTS > 15sTS > SS > CT. The volume collected by 60sTS exceeded precorneal tear volume. Ofloxacin mass followed the order 60sTS approximately 15sTS > SS > CT. Tear concentrations (mean +/- SD; N = 12) were 3.28 +/- 3.76 microg/g for 60sTS, 10.3 +/- 10.0 microg/g for 15sTS, 9.75 +/- 8.04 microg/g for SS, and 5.83 +/- 3.35 microg/g for CT. In vitro, SS, 15sTS, and 60sTS yielded accuracies of 103-107% and coefficients of variation (CV) < 9%. CT was only 85% accurate with a CV of 31%, indicating incomplete extraction during analysis. We concluded from this study that: 1) rabbit tear sampling by SS was rapid, easy, accurate, precise, and easily analyzable; 2) sampling by CT or 15sTS was accurate, but may require aggressive extraction (for CT) or be affected by tear flow rate (for 15sTS); and 3) tear sampling by 60sTS underestimated actual tear concentrations.     

Comparison of the urinary metabolite profile of caffeine in young and elderly males.

The urinary metabolite profile of caffeine was compared in a group of seven healthy young men aged 18-29 years and in a group of five healthy elderly men aged 66-71 years. All subjects were given 5 mg/kg doses of caffeine as an aqueous oral solution or an intravenous infusion on two separate occasions in a randomized crossover design. Urine samples were collected for 24 h after dosing and analysed for caffeine and eleven of its metabolites by high-performance liquid chromatography. The effects of age, route of administration, and order of administration by route on the metabolite profile of caffeine were examined. The route of administration and the order of administration by the two routes were found not to influence the urinary metabolite pattern significantly. The urinary metabolite profile did not vary substantially with age except for the observation that significantly greater amounts of 1-methyluric acid, 7-methyluric acid and 1,7-dimethyluric acid were excreted by the elderly subjects.     

Systemic pharmacokinetics of acetylenic retinoids in rats

In order to widen the therapeutic index of retinoids, one approach is to synthesize retinoids with reduced systemic distribution. Sixteen acetylenic retinoids were evaluated for their systemic disposition kinetics in rats after iv doses. Four pharmacokinetic parameters (i.e., total body clearance, volume of distribution at steady state, mean residence time, and the elimination half-life) were calculated for all retinoids tested. These compounds were categorized into four groups according to their functional head group. Retinoic acids having the trimethylcyclohexenyl head group as isotretinoin most mimicked isotretinoin in disposition profiles among all retinoic acids examined. They had volumes of distribution similar to and mean residence times shorter than those of isotretinoin. Retinoic acids containing the tetramethyltetralinyl head group as arotinoid had extensive tissue distribution and small body clearance. They had extended elimination half-lives similar to those observed for etretinate. Dimethylchromanyl and dimethylthiochromanyl retinoic acids were more polar; their terminal half-lives were reasonably short and no extensive tissue distribution was noted. The ethyl retinoates rapidly converted to their corresponding retinoic acids after iv doses. All ethyl esters had limited systemic residence times. The ethyl nicotinates tended to have much larger body clearance (10- to 25-fold) than the ethyl benzoates. After iv administration of ethyl retinoates, the ethyl esters disappeared rapidly, while their corresponding retinoic acids became the major drug-derived species in blood. The study results demonstrated different pharmacokinetic behaviors of acetylenic retinoids with different functional head groups.     

Systemic pharmacokinetics of acitretin, etretinate, isotretinoin, and acetylenic retinoids in guinea pigs and obese rats.

Etretinate, a highly lipophilic retinoid, is known to accumulate in the human body with a slow systemic elimination (half-life approximately 100 days) after long-term treatment. Retinoids with high lipophilicity and slow body elimination have the propensity of eliciting teratogenic effects. Therefore, synthetic retinoids with reduced systemic retention are desired. In this study, we evaluated the systemic pharmacokinetics of acitretin, etretinate, isotretinoin, synthetic acetylenic retinoic acids (AGN 190121, AGN 190186, and AGN 190299), and acetylenic retinoates (AGN 190073, AGN 190089, and AGN 190168) in guinea pigs following iv doses. Their pharmacokinetics were also measured in obese rats to probe the effect of body fat on the drug disposition of retinoids. The acetylenic retinoates were hydrolyzed to their corresponding free acids at a much faster rate than etretinate in both animal species. All retinoates showed faster body clearance and larger volume of distribution than their free acids. In the obese rats, longer elimination half-lives and slower body clearance of the retinoids, except isotretinoin, were observed as compared to those in the normal rats. These results suggest that body fat has a significant effect on drug disposition and slows down the systemic clearance of retinoids. Since the synthetic acetylenic retinoates rapidly converted to their less lipophilic free acids after systemic absorption, the potential accumulation of these retinoids, as reported for lipophilic etretinate, were unlikely to occur in humans and animals.     

Comparison of Azone and Hexamethylene Lauramide in Toxicologic Effects and Penetration Enhancement of Cimetidine in Rabbit Eyes

Pharmaceutical Research -     

The use of precision-cut rat lung slices for studying PGF(2α) metabolism

The suitability of a dynamic lung slice culture system as an in vitro model for studying pulmonary metabolism of PGF_2α was assessed. [³H]Prostaglandin F_2α ([³H]PGF_2α), a twenty carbon fatty acid that contains a five-carbon ring and is known to be metabolized by lung in vivo, was incubated with precision-cut rat lung slices in 1.7 ml of Waymouth's buffer fortified with 10% fetal calf serum. At 0, 2, 4 and 8 h after addition of [³H]PGF_2α (1.82 ng/μCi), incubation was stopped and the contents of each vial were analyzed for [³H]PGF_2α and its metabolites using reversed-phase HPLC with radiochemical detection. PGF_2α was metabolized to 15-keto PGF_2α, 13,14-dihydro-15-keto PGF_2α, and two unknown minor polar metabolites. These results indicate that PGF_2α was metabolized in lung slices pathways similar to those seen in vivo. Slice viability was assessed by protein synthesis and light microscopic examination of lung slices through 24 h of incubation. Protein synthesis was maintained and no tissue necrosis was observed over the entire 24 h incubation, indicating that the lung slices were viable for at least 24 h. These results indicate that the dynamic lung slice culture system is an appropriate in vitro model for studying the pulmonary metabolism of PGF_2α.