Both naturally occurring and semi-synthetic calanolide compounds are potent anti-human immunodeficiency virus (HIV) agents. In fresh human cells, they are highly effective inhibitors against low passage clinical virus strains, including those representative of the various HIV-1 clade strains (A through F), syncytium-inducing (SI) and non-syncytium-inducing (NSI) isolates, and T-tropic and monocyte-tropic isolates. These compounds also exhibit an enhanced antiviral activity against one of the most prevalent non-nucleoside reverse transcriptase inhibitor (NNRTI)-resistant viruses that is engendered by the Y181C amino acid change in reverse transcriptase (RT). Further enhancement of activity is observed with RTs that possess the Y181C change together with AZT-resistant mutations. Moreover, when challenged with viruses containing Y181C and K103N dual mutations, calanolide compounds remain active. These dual mutations are highly resistant to all approved NNRTIs (eg, delavirdine, nevirapine and efavirenz). In cell culture assays, calanolide compounds, especially (+)-calanolide A, select primarily resistant viruses possessing the T139I amino acid change. This mutation appears to be unique to calanolides since it remains susceptible to other NNRTIs. Synergistic effects are observed in both cultured cells and animal models when calanolides are used in combination with other anti-HIV agents. Enzymatic analyses indicate that calanolides inhibit HIV-1 RT through a mechanism that affects both the Km for normal substrate dTTP and the Vmax, resulting in a mixed-type inhibition, which is different from that of other known NNRTIs. Two possible binding modes/sites at the HIV-1 RT enzyme have been suggested for (+)-calanolide A. Taken together, the calanolide compounds represent a novel and distinct subgroup of the NNRTI family and inclusion of a calanolide in a combination therapy may be clinically beneficial. Of particular interest is the use of calanolide in the treatment of patients who have failed other NNRTI therapy and developed the Y181C mutation or the Y181C/K103N dual mutations. Currently, (+)-calanolide A, the most potent in the series of calanolide compounds, is undergoing clinical investigation for safety and efficacy in HIV-infected individuals.
A HPLC method was validated for quantification of (+)-calanolide A (1), a novel anti-HIV agent, in rat, dog and human plasma. The synthetic intermediate (+/-)-12-oxocalanolide A (2) was found to be a suitable internal standard. Compounds were extracted from plasma using a solid-phase C(18) cartridge and quantified over the assay range of 12.5 to 800 ng/ml. The method was utilized to determine (+)-calanolide A pharmacokinetics in rats, dogs and humans. This is the first report of a validated HPLC assay for determination of (+)-calanolide A concentrations in rat and dog plasma as well as human plasma obtained from clinical trials. There was no evidence of in vivo epimerization of (+)-calanolide A to its inactive epimer (+)-calanolide B (3).
Plant-derived and semi-synthetic calanolide compounds with anti-human immunodeficiency virus type 1 (HIV-1) activity were tested for anti-human cytomegalovirus (HCMV) activity in both cytopathic effect inhibition and plaque reduction assays. The results indicated that the anti-HCMV activity of calanolide compounds does not correlate with their activity against HIV-1. The semi-synthetic 12-keto derivatives tended to be more active against HCMV than the corresponding 12-OH congeners, which were more active against HIV-1. It appeared that the 7,8-unsaturated double bond in the chromene ring played a certain role in maintaining activities against both HCMV and HIV-1. Saturation of the double bond increased the EC50 values against both viruses, with concomitant increase in toxicity. The calanolide compounds reported here are the first non-nucleoside analogues capable of inhibiting both HIV-1 and HCMV and, therefore, may be useful chemoprophylactic agents for HCMV in HIV-infected people or vice versa.