Susceptibility testing of 68 cytomegalovirus (CMV) peripheral blood isolates to
Ganciclovir (
DHPG) and 11 blood isolates to
Foscarnet (PFA), was performed on primary culture isolates using the shell vial assay methodology (SVA-IFA, that is, quantitation of fluorescent focus units, FFUs), with an anti-CMV
monoclonal antibody to the late
viral antigen. A positive reaction in monolayer cultures of MRC-5 cells was characterized by cytoplasmic fluorescence with inclusions at both or more commonly off one end of the elongated fibroblast nucleus. Isolates from conventional MRC-5 tube cultures displaying a 1+ (10% cytopathic effect) were inoculated into shell vials containing
DHPG concentrations of 0, 1.5, 3, 6, 12, or 24 microliters/ml shell vials containing 400, 500, 800, or 1200 microM PFA. The optimal readability of monolayers (expressed as FFUs per monolayer) occurred at 96 h
after treatment with
DHPG and at 36-48 h with PFA. Resistance to
DHPG was determined at the concentration of
antiviral agent necessary to reduce the number of FFUs to 90% or 50% of the control [that is, the 90% minimum inhibitory concentration (MIC90) or MIC50]. Six of 68 isolates showed an MIC90 > 12 or an MIC50 > 1.5 microgram/ml, and were considered
DHPG resistant. Three of the six isolates were from
AIDS patients with late-stage disease who had never received
DHPG therapy. All but one (specimen 2400)
DHPG-resistant isolates revealed MIC90 values to a PFA concentration of 500 microM, which is considered an achievable peak plasma level in patients undergoing PFA
therapy. The single
DHPG- and FPA-resistant isolate was obtained from a patient displaying marked clinical resistance to both drugs.(ABSTRACT TRUNCATED AT 250 WORDS)