Quantitative Analysis of Flesinoxan in Plasma and Urine at the pg/ml Level Using GC-ECD

Ceelan et al. [1] developed an HPLC method for the determination in plasma of flesinoxan (given as the hydrochloride; DU 23973), a relatively large and polar molecule of low volatility, shown on left in the diagram below. However, on the basis of the results of the first safety and tolerance study the dose was lowered to 0.25 mg/person, resulting in plasma levels in the pg/ml range. For the pharmacokinetic monitoring of the drug at this low dose, the HPLC method was unsuitable. A new analytical procedure had to be developed using a completely different strategy aiming at utmost sensitivity. GC-ECD⊗ is a highly sensitive technique which could be suitable after hydrolysis of the drug to the FBz and then esterification with PFBB