Co-Trimoxazole and Stavudine Interference in a High-Performance Liquid Chromatographic Analysis for Didanosine in Human Plasma

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Abstract
Recently, the authors were confronted with interference of stavudine and co-trimoxazole when analyzing the antiretroviral drug didanosine (ddI) in plasma of HIV-1-infected patients using reverse-phase high-performance liquid chromatography with ultraviolet detection. After increasing the percentage of methanol in the mobile phase from 4% to 8% vol/vol and after decreasing the pH of the mobile phase from 6.8 to 5.8, the authors were able to separate didanosine from stavudine and co-trimoxazole (both are frequently used drugs in combination with didanosine). Subsequently, the adapted bioanalytic methodology was validated, and validation results showed that this new methodology can be used for the quantitative determination of didanosine in human plasma. This observation makes clear that combination therapy for human immunodeficiency virus with multiple (often chemically related) drugs has the potential of unexpectedly complicating bioanalytic analyses because therapeutic strategies may change rapidly after publication of a bioanalytic methodology. Thus, it is evident that the investigation of interference of potentially coadministered drugs should be a standard procedure during the development of any bioanalytical methodology in any laboratory.