The selective inhibition of viral DNA synthesis by chemotherapeutic agents: an indicator of clinical usefulness?

A cell culture system has been utilized to measure the effects of drugs on DNA synthesis in uninfected and HSV-(herpes simplex virus)-infected KB cells. DNA from HSV-infected cells was separated into viral and cellular components by isopycnic centrifugation in CsCl gradients. The amount of [3H]thymidine incorporated into acid-insoluble material was measured in the absence and presence of drugs. Dose-response relationships were established by linearly regressing the probit value of the percent inhibition DNA synthesis against the logarithm of drug concentration. Fifty percent inhibitory (I50) concentrations were interpolated from the corresponding regression lines for inhibition of the following: (i) DNA synthesis is uninfected KB cells, (ii) total DNA synthesis in HSV-infected KB cells (iii) cellular DNA synthesis in HSV-infected cells, and (iv) viral DNA synthesis in HSV-infected cells. We have derived an index (SI, selective index) that quantifies the preferential inhibition of viral or uninfected cellular DNA synthesis. This index can be expressed as SI = log10 I50 concentration for DNA synthesis in uninfected cells divided by I50 concentration for viral DNA synthesis in HSV-infected cells. The SI is positive if viral DNA synthesis is inhibited preferentially and negative if uninfected cellular DNA synthesis is more strongly inhibited. A positive SI value of 0.5 was obtained for the clinically useful antiviral drug arabinosyladenine (ara-A) and a value of 0.4 for its metabolite, arabinosylhypoxanthine (ara-H). Although the adenosine deaminase inhibitor coformycin greatly increased the potency of ara-A, the inhibitor did not increase the selectivity of the drug (SI = 0.3). Stallimycin (distimycin A) (SI = 0.3) and phosphonoacetic acid (SI = 0.3) were similarly effective in preferentially inhibiting the synthesis of HSV DNA. In contrast, arabinosylcytosine (ara-C) and ribavirin inhibited DNA synthesis in uninfected cells to a greater degree than viral DNA synthesis (SI = -0.5 and -1.9, respectively). An analysis of the advantages and limitations of this experimental procedure is made and the suggestion is offered that the in vitro determination of a drug's selective index may be a valid predictor of clinical usefulness.