Wiersema P h a r m a c e u t i c a l L a b o r a t o r y , D e p a r t m e n t o f A n a l y t i c a l Pharmacy U n i v e r s i t y o f U t r e c h t , C a t h a r i j n e s i n g e l 6 0 , 3511 GH U t r e c h t , The N e t h e r l a n d s ABSTRACT A r a d i a l l y c o m p r e s s e d column, p a c k e d w i t h micro p a r t i c l e , r e v e r s e d p h a s e ( C 1 8 ) m a t e r i a l , w a s u s e d t o s t u d y HPLC w i t h c e t r i m i d e c o n t a i n i n g e l u e n t s . The amount o f c e t r i m i d e a d s o r b e d o n t o t h e s t a t i o n a r y p h a s e w a s measured: n o t t h e number o f a v a i l a b l e a d s o r p t i o n s i t e s , b u t r a t h e r t h e p r e s e n c e o f m i c e l l e s i n t h e e l u e n t a p p e a r s t o be t h e l i m i t i n g f a c t o r € o r t h e u p t a k e of c e t r imide from t h e e l u e n t . compounds were d e t e r m i n e d i n t h i s s y s t e m , w i t h v a r y i n g pH and c e t r i m i d e c o n c e n t r a t i o n of t h e e l u e n t ( m e t h a n o l -w a t e r , 5 0 % w / w ) . The r e s u l t s o b t a i n e d upon c h a n g i n g t h e pH of t h e e l u e n t c a n n o t a l l b e e x p l a i n e d w i t h a n i o n -e x c h a n g e ( o r i o n -p a i r ) model. Upon i n c r e a s i n g t h e c e t r i m i d e c o n c e n t r a t i o n , maximum v a l u e s i n k ' a r e r e a c h e d a t a b o u t t h e c r i t i c a l m i c e l l e c o n c e n t r a t i o n ( c m c ) of c e t r i m i d e i n t h e e l u e n t . The res u l t s o f c o n d u c t i m e t r i c e x p e r i m e n t s s u g g e s t e d , t h a t t h e d e c r e a s e o f k ' a t c e t r i m i d e c o n c e n t r a t i o n s a b o v e t h i s CKC, o b s e r v e d f o r m o s t o f t h e compounds, c a n b e e x p l a i n e d by p a r t i t i o n i n g
Nalidixicacid (l-ethyl-l,4-dihydro-7-methyl~-oxo-l,8-naphthyridine-3-carboxylic acid; NA) is used in the treatment of urinary-tract infections caused by Gram negative organisms other than Pseudomonas spp.Most methods for its quantitative analysis are based on the fluorescence of NA [1, 21, but these methods have several disadvantages. They are nonspecific, giving relatively high blank values and non-linear calibration curves and they need relatively large plasma volumes of 1-3 ml. A high-performance liquid chromatographic (HPLC) method using an ion-exchange column has been described 131, which needs only 1 ml of plasma, but in this method no internal standard is used. It is suggested that with this method the major metabolite, hydroxynalidixic acid (I-ethyl-1,4-dihydro-7-hydroxymethyl-4-oxo-l,Snaphthyridine-3-carboxylic acid; HNA) can also be determined, but this compound is not separated from the solvent peak in the system used.In this paper we describe a method that can measure NA concentrations down to 0.6 pg/ml in a loo-cl1 plasma sample using a standard reversed-phase column. Because nalidixic acid gives very strongly tailing peaks in most chromatographic systems, the compound is methylated as described in the literature 141. The calibration curve is linear up to at least 100 Dg/ml and the intercept does not differ significantly from zero. Also the metabolite does not interfere with the assay. The method was used for determining plasma levels in patients treated with nalidixic acid.
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