The availability of antiretroviral therapy (ART) has led to a reduction in morbidity and mortality in patients with chronic HIV infection. ART, however, is not free of side effects, has potential pharmacological interactions with other drugs, is expensive and can be complex, especially in those patients with high therapeutic experience and archived antiretroviral drug resistance mutations (DRM). For this reason, the development of simplified and equally effective therapeutic strategies that save patients from exposure to antiretroviral agents remains of great interest, even in patients with DRM and limited therapeutic options.

Currently, Darunavir (DRV) in combination with either ritonavir (RTV) or cobicistat (COBI) is the most widely recommended boosted protease inhibitor. It is used in most clinical settings, including those with limited therapeutic options and highly ART-experienced patients carrying multiple DRM. In addition, DRV has good tolerance and safety profiles, a high genetic barrier and can be administered once daily in patients harboring DRM in the viral protease with little or no impact on viral sensitivity (1). As a booster, RTV has an inducing effect on glucuronidation and a broad and potent inhibitory effect on cytochrome P-450 (CYP) isozymes and drug transporters, resulting in a significant number of drug interactions. A low dose of RTV does not appear to cause substantial antiviral activity, although its theoretical contribution to the emergence of drug resistance is still unclear. By contrast, the metabolism of COBI is predominantly via CYP3A4 oxidation and, to a lesser degree, CYP2D6. COBI does not undergo glucuronidation. In addition, 99% of COBI remains unchanged, and the resulting metabolites do not seem to show any clinically relevant inhibitory activity.

Dolutegravir (DTG) is the latest available agent within the antiretroviral class of integrase strand-transfer inhibitors (INSTI). In this group, DTG is the drug with the greatest genetic barrier, derived from its greater affinity for integrase and its consequent longer dissociation time of the drug-integrase complex. The development of DTG resistance associated mutations reported in the clinical setting has been purely anecdotal, and it has not been observed in clinical trials in naïve patients. Moreover, DTG retains the ability to inhibit viral replication when integrase associated mutations have been selected by other INSTI-based treatments (i.e., raltegravir and elvitegravir/cobicistat). DTG may also favor therapeutic adherence due to its high tolerability and easy administration with once or twice daily dosages depending on the absence or presence of mutations in the integrase or prior failures with other INSTIs. Finally, DTG is eliminated mainly through metabolism by UGT1A1, and it is also a substrate of UGT1A3, UGT1A9, CYP3A4, Pgp, and BCRP. Therefore, all drugs that induce these enzymes may decrease DTG's plasma concentration and reduce its therapeutic effect. In one study, coadministration of DRV/rtv (600/100 mg twice daily) and DTG (30 mg once daily) decreased DTG Cmax, AUC and Ctrough by 11%, 22% and 38%, respectively (2). Despite these changes, it is assumed that DRV/rtv has no clinically significant effect on the pharmacokinetics of DTG and no dose adjustment is recommended.

It has been established that it is necessary to have at least two active drugs within the optimized ART to achieve and maintain virological suppression in multi-treated patients with multiple DRM. In addition, some studies have shown that recycled NRTIs or NNRTIs with residual activity are not necessary and can be withdrawn from ART optimized regimens in extensively pretreated patients with sustained virological suppression (3). In fact, limited data from some observational and clinical studies further suggest that bitherapy as simplification strategy could be also safe and effective (4). Recently, an observational study performed on a limited and heterogeneous number of highly pretreated patients who were switched to DRV/rtv plus DTG reported efficacy rates of virological suppression of up to 98% at 48 weeks (5). In addition, there are two ongoing studies (NCT02491242 and NCT02486133) that are currently evaluating bitherapy consisting of DTG and DRV. The first of these studies is an observational study with no comparator arm which is evaluating the efficacy of DTG-based bitherapy, including RPV, 3TC, or boosted DRV (both with RTV and COBI) as the second agent. The second one is a clinical study comparing the switch to DTG plus DRV/rtv vs. triple ART based on DRV/rtv in patients with limited ART experience and virological suppression. Due to the greater specificity of COBI for CYP3A4 and its lack of effect on glucuronidation in comparison to RTV, DTG plus DRV/cobi bitherapy would constitute a simplification strategy for ART consisting of two drugs with well-known efficacy, safety, tolerability, high genetic barrier, once-daily administration, and relatively less potential for pharmacological interactions than traditional optimized based regimens. As far as the investigators know, however, there are no pharmacokinetic data and clinical trials specifically evaluating DRV/cobi plus DTG as a simplification strategy.