Development of a radioimmunoassay for latanoprost and its application in a long-term study in monkeys

13,14-Dihydro-17-phenyl-18,19,20-trinor-PGF2 alpha-isopropyl ester (latanoprost) is a new prostaglandin drug developed for the treatment of glaucoma. In clinical trials a daily dose of 1.5 micrograms is effective in reducing the intraocular pressure. In toxicological studies doses from 2 micrograms/eye to 100 micrograms/eye have been used in various species. This paper reports the development and validation of a radioimmunoassay of latanoprost acid (PhXA85) and its application to toxicokinetic studies performed in monkeys. An antiserum was raised in rabbits by immunization with PhXA85 coupled to BSA at the carboxylic acid by the mixed anhydride method. The antibody titre was found to be about 1:2000 to 1:3000. The cross-reactivity with 13,14-dihydro-15(R,S)-17-phenyl-trinor-PGF2 alpha, 13,14-dihydro-15(S)-17-phenyl-trinor-PGF2 alpha, dinor-PhXA85. 17-phenyl-trinor-PGF2 alpha, latanoprost and PGF2 alpha was 46.4, 4.2, 7.6, 2.2, 0.1 and 0.039%, respectively. The intra-assay precision was between +/-7.7 and 11.7% (CV) at the level of 320 pg/ml and +/-8.3 and 9.7% with 1280 pg/ml in plasma samples from man, monkey, rat and aqueous humour from human and rabbit. Similarly, the intra-assay accuracy varied between 95.9 and 102.5% and 89.0 and 109.0% for the low and high standards, respectively. The inter-assay precision and accuracy were between +/- 6.0 and 13.4% and 91.0 and 92.8% in the monkey plasma samples. The limit of detection was 3 pg/tube or 30 pg/ml. In a long-term study, the acid of latanoprost was rapidly cleared from plasma in monkeys treated with eye drops of latanoprost (2 x 3 micrograms/day) over a period of 1 year.