Tipranavir Without Ritonavir Does Not Acutely Induce Peripheral Insulin Resistance in a Rodent Model

To the Editor: The clinical use of HIV protease inhibitors (PIs) is known to be independently associated with the development of insulin resistance.1 Not all PIs seem to have the same propensity to cause this effect, however. Although ritonavir and indinavir have been associated with the greatest degree of altered glucose homeostasis,2,3 in vitro, animal, and clinical data indicate that atazanavir does not produce alterations in peripheral glucose disposal.4,5 Our previous studies have also shown excellent correlation between the acute effects of these drugs on peripheral glucose disposal in rodents subjected to hyperinsulinemic euglycemic clamp studies and effects seen in humans, clinically and in healthy HIV-negative human volunteers.4 These studies have supported the contention that investigation of the acute effects of PIs in this model system can reliably predict the potential for PIs to contribute significantly to insulin resistance in treated patients. Insight into the molecular basis of the ability of PIs to induce insulin resistance has recently been provided by a careful analysis of the shared structural features found within each of the PIs in clinical use. We have previously shown that the hydrophobic peptidomimetic backbone that confers antiretroviral activity directly contributes to the inhibitory effects of these drugs on the intrinsic activity of the insulin-responsive facilitative glucose transporter GLUT4.6 This conclusion is partially based on the discovery that small oligopeptides containing this core structure act similarly to inhibit GLUT4 activity in vitro. Although it has not been definitively established why atazanavir is unique in its ability to inhibit the HIV protease without directly blocking GLUT4 activity, the additional pyridine ring attached to one of the phenylalanine-like residues and/or the lack of the same degree of flanking hydrophobicity may be responsible for this effect. Tipranavir, the newest of the PIs approved for clinical use in the United States, differs from other drugs in this class in that it does not contain a peptidomimetic core structure. Although insulin resistance is listed as a potential side effect of this drug,7 the limited clinical experience in using tipranavir has not allowed a full assessment of its relative ability to alter glucose disposal compared with other PIs. The ability to predict whether and to what degree tipranavir affects glucose homeostasis can assist clinicians in deciding which patients are best suited to the introduction of this drug as part of highly active antiretroviral therapy. To examine the ability of tipranavir to induce insulin resistance acutely, we subjected healthy, lean (215-225 g), HIV-negative, male Wistar rats to hyperinsulinemic euglycemic clamps as previously described.8 All animals had catheters surgically implanted into the jugular vein and external carotid arteries at least 5 days before clamp experiments. A continuous infusion of tipranavir, tipranavir plus ritonavir, or ritonavir (0.35 and 0.14 mg/kg/min for tipranavir and ritonavir, respectively) was initiated 30 minutes before the start of insulin administration (10 mU/kg/min). In both of the ritonavir-treated groups, the drug infusion rates were reduced to 0.28 and 0.112 mg/kg/min for tipranavir and ritonavir, respectively, after 15 minutes to maintain steady-state tipranavir levels comparable to the tipranavir-alone group. Blood sugars were monitored every 5 minutes from the arterial catheter. Glucose was simultaneously infused into the jugular vein at a variable rate to maintain serum glucose at 105 mg/dL. Serum was removed during the steady-state phase of the clamp (30 and 90 minutes) for the determination of tipranavir and ritonavir levels by high-performance liquid chromatography (HPLC) analysis9 using pure drug standards from the National Institutes of Health (NIH) AIDS reagent reference program. The mean steady-state insulin and glucose levels in each of the treatment groups were not statistically different. As reported in Table 1, no difference in the steady-state rate of glucose disposal (R′d) was observed in tipranavir-treated animals compared with vehicle-treated (normal saline) animals. The steady-state tipranavir levels achieved in this experiment are near the peak of serum levels achieved in treated patients.7 Infusion of ritonavir alone resulted in a 45% reduction in R′d, with sustained serum drug levels of 6.2 μM. The R′d was not further reduced when tipranavir was infused together with ritonavir at a ratio of 2.5:1, however.TABLE 1: Acute Effects of Tipranavir and/or Ritonavir on Peripheral Glucose Disposal in Healthy Lean RodentsThese data indicate that tipranavir alone is not likely to alter peripheral insulin resistance acutely in treated patients. The concomitant administration of ritonavir together with tipranavir, as in boosted antiretroviral regimens, can adversely affect insulin sensitivity, however. It remains to be determined whether chronic tipranavir exposure influences insulin sensitivity through indirect mechanisms such as altered adipocytokine production or the development of hyperlipidemia. Nevertheless, the determination that this nonpeptidomimetic PI does not contribute to altered glucose disposal offers continued hope that additional PI-containing antiretroviral regimens that do not acutely block glucose transport can be identified. These efforts should be aided by the development of PIs that do not require ritonavir boosting. Continued use of this healthy rodent model system to assess the ability of new drugs to induce insulin resistance directly can allow efficient screening of candidate compounds before the initiation of large and expensive clinical trials. Paul W. Hruz, MD, PhD Departments of Pediatrics and Cell Biology and Physiology Washington University School of Medicine St. Louis, MO Qingyun Yan, MD Department of Pediatrics Washington University School of Medicine St.