Simultaneous determination of the new HIV protease inhibitor lopinavir (ABT 378) and of indinavir(1), amprenavir, saquinavir, ritonavir (ABT 538)(2) and nelfinavir(3) in human plasma by gradient HPLC.

Protease inhibitors are known by their inhibition of a viral protease that leads to production of immature and non-infectious virus particles. The novel protease inhibitor KALETRA is a co-formulation of lopinavir and ritonavir. Ritonavir reduces the metabolization of lopinavir by the cytochrome P450 3A4 isoenzyme which leads to markedly increased plasma levels of lopinavir(4). A new rapid and sensitive HPLC method for the simultaneous determination of lopinavir, indinavir, amprenavir, saquinavir, ritonavir and nelfinavir in human plasma has been developed. An aliquot of 500 microl plasma, spiked with internal standard, was extracted with 500 microl 0.1 M ammonium hydroxide solution and 5 ml tert. -butyl ether. After drying under a nitrogen stream, the residue was redissolved in an eluent consisting of 50 mM phosphate buffer, pH 5.40 and acetonitrile (50:50, v/v). Chromatographic separation was accomplished on a C-18 column using a non-linear gradient elution and ultraviolet detection at 215 nm.