Development of resistance of zidovudine (ZDV) and didanosine (ddI) in HIV from patients in ZDV, ddI and alternating ZDV/ddI therapy.

OBJECTIVE

To study development of phenotypic and genotypic resistance against zidovudine (ZDV) and didanosine (ddI) during 24 months of mono- and monthly alternating therapy.

PATIENTS

Forty-six patients, not previously treated with antiretroviral drugs, were included in the study.

METHODS

ZDV and ddI sensitivity were determined in a biological assay based on production of HIV antigen in cultures of CD4+ lymphocytes. The ZDV-associated mutations at codon 41 and 215, and the ddI-associated mutation at codon 74 of the reverse transcriptase (RT) gene were analysed using selective polymerase chain reaction on DNA from peripheral blood mononuclear cells. The biological phenotype [syncytium-inducing (SI)/non-SI(NSI)] of the viral isolates was assessed using a MT2 assay.

RESULTS

Of the patients, 82% in ZDV therapy and 73% in alternating therapy developed phenotypic resistant HIV [median inhibitory concentration (IC50) > 0.1 microM]. Patients treated for 1 year with ddI (monotherapy or alternating) had significant higher ddI IC50 values than patients in ZDV monotherapy. During ZDV and alternating therapy, 67 and 75% of the patients, respectively, developed mutations in RT codon 41, whereas 83 and 75%, respectively, developed mutations in codon 215. In patients treated with ddI, 60% developed mutations in codon 74, whereas none of the patients in either alternating ZDV/ddI or ZDV therapy developed this mutation. Forty-six per cent of the patients had SI HIV at start of therapy. Four patients switched from SI to NSI during either ZDV, ddI or alternating therapy. Faster development of resistance was associated with the SI phenotype.

CONCLUSIONS

No difference in either phenotypic ZDV or ddI resistance, or genotypic ZDV resistance could be demonstrated during monotherapy or monthly alternating ZDV/ddI therapy, whereas genotypic ddI resistance (mutation in RT codon 74) only were detected in patients in ddI monotherapy. In addition, we found that development of phenotypic and genotypic resistance was faster in patients harbouring SI isolates, and that switches from SI to NSI during therapy was independent of the type of therapy.