We found that single doses of the PIs ritonavir and amprenavir had different effects on insulin sensitivity as measured by insulin-mediated glucose disposal, although a similar reduction of FFA levels by insulin were observed on both drugs. Full-dose ritonavir caused a 15% reduction in insulin-mediated glucose disposal whereas amprenavir had little effect. Previously we have reported that a single dose of indinavir in therapeutic range decreased insulin-mediated glucose disposal by 34%. We have also reported that a single dose of lopinavir/ritonavir decreased insulin-mediated glucose disposal by 13% [6
]. All four studies were performed under identical study conditions with participants of similar ages and ethnic composition. While direct comparison of four drugs was not possible in single study, the identical study design in similar study populations permits a relative comparison of the effects of each PI versus placebo. Taken together, we found that indinavir had the greatest effect on insulin sensitivity as measured by insulin-mediated glucose disposal. Full-dose ritonavir and lopinavir/ritonavir had similar effects on insulin-mediated glucose disposal. In contrast, amprenavir did not alter insulin-mediated glucose disposal.
The single dose of ritonavir was given only 2 h before the study, suggesting that ritonavir acutely induced insulin resistance. These findings are consistent with an acute and immediate blockade of the peripheral glucose transporter, GLUT4, as has been demonstrated previously [5
]. This decrease in insulin-mediated glucose disposal was mostly due to a defect in peripheral glucose storage, as reflected in the decline in the rate of non-oxidative glucose disposal. Both indinavir and lopinavir/ritonavir also acutely decreased non-oxidative, but not oxidative glucose disposal [4
Free fatty acids were suppressed to comparable levels. The ability of insulin to inhibit lipolysis during the euglycemic hyperinsulinemic clamp with ritonavir administration suggests that insulin signaling was not globally impaired, a finding similar to that with indinavir. The increase in fasting insulin 2 h after administration of ritonavir raises the possibility that hepatic insulin sensitivity is affected. We previously have shown that 4 weeks of indinavir increases endogenous glucose production in the fasting state in addition to decreasing insulin sensitivity during the clamp [12
In both studies, plasma levels of ritonavir and amprenavir closely resembled those observed in pharmacokinetic studies of healthy normal volunteers. During the last hour of the clamp, ritonavir drug levels were 8.7±0.9 to 9.0±1.0 mg/l, a range that is similar to full-dose ritonavir drug levels achieved in pharmacokinetic studies [7
] and is 2.2-fold higher than the Cmin
for ritonavir. Amprenavir levels peaked at the start of the clamp followed by a decline to levels of greater than 2.0±0.8 mg/l in a pattern typical of amprenavir kinetics in healthy normal volunteers and HIV-infected patients [8
] and is more than eight-fold higher than the Cmin
for amprenavir. The single 1200 mg dose of amprenavir is known to have a biphasic pattern by which drug levels remain at less than 2 mg/l during 18 h of the once-daily dosing. At such levels, we found that insulin-mediated glucose disposal did not change. A very small reduction in insulin sensitivity may not have been detected in the amprenavir study; however, such a small reduction in insulin sensitivity may not be clinically significant. Due to the transient nature of the peak levels of amprenavir, it was not possible to measure insulin-mediated glucose disposal at peak amprenavir levels; however, HOMA-IR at peak amprenavir levels at the start of the clamp study also did not change (). In contrast, the single dose of ritonavir increased HOMA-IR at the start of the clamp study and decreased insulin-mediated glucose disposal. These findings suggest that amprenavir has little effect on altering insulin sensitivity at either peak or average therapeutic levels. These results are for single dose amprenavir, and it is possible that amprenavir given twice daily may still have an effect on insulin resistance.
Differences in the standard therapeutic drug levels and in the solubility of these drugs in aqueous solution may partly explain the disparities in results between human, animal, and in vitro
studies. In a comparative study of the effects of 50 μmol/l indinavir, ritonavir, and amprenavir on glucose uptake in 3T3L-1 adipocytes, ritonavir caused a striking inhibition of 2-deoxy glucose uptake, followed by a more modest inhibition by amprenavir and indinavir [14
]. These levels used in vitro
in an assay without binding proteins, however, exceed peak plasma levels and do not take into account free drug levels. In a follow-up study, rats were infused intravenous amprenavir, ritonavir, or lopinavir/ritonavir to achieve drug levels of 8–12 μmol/l, and euglycemic hyperinsulinemic clamps were performed [15
]. Interestingly, a similar rank order of insulin sensitivity was observed in rats (ritonavir followed by lopinavir/ritonavir and amprenavir) as in our study. At 11.3 μmol/l of amprenavir, there was an 18.4% decrease in insulin-mediated glucose disposal in rats, but the drug levels achieved in rats were much higher than those observed with typical pharmacokinetics in humans. As free drug levels may differ between rats and humans, it is difficult to extrapolate the direct effects on humans in the absence of clinical studies. In our study, drug levels were within therapeutic range for amprenavir. This suggests that the biphasic pharmacokinetics of amprenavir in humans may be advantageous in maintaining drug concentrations at levels that do not induce insulin resistance.
Careful consideration should be given to interpreting these results in the current clinical setting due to changes in dosing practices and formulations over time. In these studies which began 4 years ago, we used full dose ritonavir, which is not currently used today. We chose to study full dose ritonavir because insulin resistance and changes in glucose metabolism were reported early in the use of PIs, when full, rather than boosting, doses were used. Ritonavir is more commonly used to boost other PIs, and it is currently not known if boosting doses of ritonavir alter insulin sensitivity. The effects of boosting doses of ritonavir are further complicated by the concomitant effects of the boosted PIs on insulin sensitivity. In the case of lopinavir/ritonavir, we previously found that a single dose decreased insulin sensitivity by 13%, but it is unclear if lopinavir, ritonavir, or both PIs are responsible for the induction of insulin resistance. Amprenavir 1200 mg soft gel capsules are no longer used and have largely been replaced by fosamprenavir, which has yet to be studied with regard to insulin sensitivity.
In summary, we report that a single dose of ritonavir decreased insulin-mediated glucose disposal in healthy normal volunteers, whereas amprenavir has little effect on insulin-mediated glucose disposal under nearly identical study conditions. These studies are placed in context of the known effects of a single dose of indinavir and lopinavir/ritonavir in identical study designs. A single dose of indinavir induced the largest decrease in insulin-mediated glucose disposal. Full-dose ritonavir and lopinavir/ritonavir had lesser effects on insulin-mediated glucose disposal whereas amprenavir had no significant effect on insulin-mediated glucose disposal. These were four separate studies in identical protocols and similar, but not identical, study populations. As commonly used dosages and formulations of PIs have evolved over time and differ from those used in this study, careful consideration should be given to application of these findings to clinical practice.