22P A D I F F E R E N T I A L ASSAY FOR CHLORAMPHENICOL AND CHLORAMPHENICOL '"SUCCINATE BASED UPON A S E N S I T I V E ASSAY FOR THE PARENT DRUG Roger T. Guest, P e t e r R. Byron, Andreas Gescher. Pharmaceutics Research Group, U n i v e r s i t y of Aston i n Birmingham, Birmingham B4 7ET. To e v a l u a t e prodrugs of a n e x i s t i n g drug i t i s n e c e s s a r y t o d i f f e r e n t i a t e between t h e p a r e n t compound and i t s b i o r e v e r s i b l e d e r i v a t i v e ( N o t a r i , 1 9 7 7 ) . A simple s e n s i t i v e a s s a y h a s been developed c a p a b l e o f q u a n t i f y i n g chloramphenicol (CAP) and chloramphenicol s u c c i n a t e (CAP-sl a t amounts g r e a t e r t h a n 0 . 5 ~ 9i n b i o l o g i c a l samples. The o p t i m i s e d a s s a y i n v o l v e s GLC of s i l y l a t e d CAP and t e t r a p h e n y l e t h y l e n e a s t h e i n t e r n a l s t a n d a r d ( I S ) u s i n g a flame i o n i s a t i o n d e t e c t o r . R e t e n t i o n times a r e 9 . 4 minutes ( I S ) and 10.8 minutes (CAP). A r e c t i l i n e a r c a l i b r a t i o n c u r v e o f peak h e i g h t r a t i o v e r s u s i n j e c t e d amount o f

CAP r e s u l t s w i t h r > 0 . 9 9 ( n = 1 2 ) . The p r e s e n c e of CAP-s and CAP breakdown p r o d u c t s do n o t i n t e r f e r e w i t h t h i s a s s a y . E x t r a c t i o n of CAP from 2 m l aqueous medium i n t o 2ml e t h y l a c e t a t e i s >99% a t 25Oc p r o v i d e d t h a t t h e aqueous c o n c e n t r a t i o n i s