Many xenobiotics including the pharmacoenhancer cobicistat increase serum creatinine by inhibiting its renal active tubular secretion without affecting the glomerular filtration rate. This study aimed to define the transporters involved in creatinine secretion, applying that knowledge to establish the mechanism for xenobiotic-induced effects. The basolateral uptake transporters organic anion transporter OAT2 and organic cation transporters OCT2 and OCT3 were found to transport creatinine. At physiologic creatinine concentrations, the specific activity of OAT2 transport was over twofold higher than OCT2 or OCT3, establishing OAT2 as a likely relevant creatinine transporter and further challenging the traditional view that creatinine is solely transported by a cationic pathway. The apical multidrug and toxin extrusion transporters MATE1 and MATE2-K demonstrated low-affinity and high-capacity transport. All drugs known to affect creatinine inhibited OCT2 and MATE1. Similar to cimetidine and ritonavir, cobicistat had the greatest effect on MATE1 with a 50% inhibition constant of 0.99 μM for creatinine transport. Trimethoprim potently inhibited MATE2-K, whereas dolutegravir preferentially inhibited OCT2. Cimetidine was unique, inhibiting all transporters that interact with creatinine. Thus, the clinical observation of elevated serum creatinine in patients taking cobicistat is likely a result of OCT2 transport, facilitating intracellular accumulation, and MATE1 inhibition.

GS-9350 is a new chemical entity under development as a potent, mechanism-based inhibitor of human cytochrome P450 3A (CYP3A) isoforms. Its intended use is to increase the systemic exposure of coadministered agents that are metabolized by CYP3A enzymes. Unlike ritonavir, which is in current clinical use for this purpose, GS-9350 is devoid of anti-HIV activity. The pharmacokinetics of GS-9350 and its efficacy in increasing systemic exposure of the probe CYP3A substrate midazolam were examined in a study involving single- and multiple-dose escalations of GS-9350 from 50 to 400 mg. Single-dose escalation from 50 to 400 mg resulted in a 164-fold increase in GS-9350 exposure, whereas multiple-dose escalation in the dosage range of 50-300 mg resulted in a 47-fold increase in exposure. GS-9350 potently inhibited midazolam apparent clearance (95% reduction), similar in effect to ritonavir 100 mg. GS-9350 was generally well tolerated at all doses, and there was no evidence of dose-limiting toxicity. Establishing proof-of-concept, GS-9350 is currently under phase II development as a potential alternative to ritonavir for use with antiretroviral agents (including the HIV integrase inhibitor elvitegravir) that are often prescribed along with a "booster" drug.

Cobicistat (3, GS-9350) is a newly discovered, potent, and selective inhibitor of human cytochrome P450 3A (CYP3A) enzymes. In contrast to ritonavir, 3 is devoid of anti-HIV activity and is thus more suitable for use in boosting anti-HIV drugs without risking selection of potential drug-resistant HIV variants. Compound 3 shows reduced liability for drug interactions and may have potential improvements in tolerability over ritonavir. In addition, 3 has high aqueous solubility and can be readily coformulated with other agents.

ObjectiveJanus kinase inhibitors (JAKinibs) are efficacious in rheumatoid arthritis (RA) with variable reported rates of adverse events, potentially related to differential JAK family member selectivity. Filgotinib was compared with baricitinib, tofacitinib and upadacitinib to elucidate the pharmacological basis underlying its clinical efficacy and safety.MethodsIn vitro JAKinib inhibition of signal transducer and activator of transcription phosphorylation (pSTAT) was measured by flow cytometry in peripheral blood mononuclear cells and whole blood from healthy donors and patients with RA following cytokine stimulation of distinct JAK/STAT pathways. The average daily pSTAT and time above 50% inhibition were calculated at clinical plasma drug exposures in immune cells. The translation of these measures was evaluated in ex vivo-stimulated assays in phase 1 healthy volunteers.ResultsJAKinib potencies depended on cytokine stimulus, pSTAT readout and cell type. JAK1-dependent pathways (interferon (IFN)α/pSTAT5, interleukin (IL)-6/pSTAT1) were among the most potently inhibited by all JAKinibs in healthy and RA blood, with filgotinib exhibiting the greatest selectivity for JAK1 pathways. Filgotinib (200 mg once daily) had calculated average daily target inhibition for IFNα/pSTAT5 and IL-6/pSTAT1 that was equivalent to tofacitinib (5 mg two times per day), upadacitinib (15 mg once daily) and baricitinib (4 mg once daily), with the least average daily inhibition for the JAK2-dependent and JAK3-dependent pathways including IL-2, IL-15, IL-4 (JAK1/JAK3), IFNγ (JAK1/JAK2), granulocyte colony stimulating factor, IL-12, IL-23 (JAK2/tyrosine kinase 2) and granulocyte-macrophage colony-stimulating factor (JAK2/JAK2). Ex vivo pharmacodynamic data from phase 1 healthy volunteers clinically confirmed JAK1 selectivity of filgotinib.ConclusionFilgotinib inhibited JAK1-mediated signalling similarly to other JAKinibs, but with less inhibition of JAK2-dependent and JAK3-dependent pathways, providing a mechanistic rationale for its apparently differentiated efficacy:safety profile.

The experimental pharmacoenhancer cobicistat (COBI), a potent mechanism-based inhibitor of cytochrome P450 3A enzymes, was found to inhibit the intestinal efflux transporters P-glycoprotein and breast cancer resistance protein. Consistent with its transporter inhibition, COBI significantly increased the absorptive flux of potential candidates for clinical coadministration, including the HIV protease inhibitors atazanavir and darunavir and the lymphoid cell-and tissue-targeted prodrug of the nucleotide analog tenofovir, GS-7340, through monolayers of Caco-2 cells in vitro.C obicistat (COBI) is being developed as a pharamacoenhancer (booster) for coformulation with drugs that are metabolized by cytochrome P450 3A (CYP3A) enzymes. Similar to ritonavir (RTV; currently used as a pharmacoenhancer of protease inhibitors [PIs] used to treat human immunodeficiency virus [HIV]), COBI is a potent mechanism-based CYP3A inhibitor, and its coadministration with CYP3A substrates can lead to desired boosting effects and unintended drug-drug interactions. COBI has been found to have a number of potentially differentiating attributes relative to RTV: (i) it has more selective CYP3A inhibition over other CYP enzymes, (ii) it has improved solubility and coformulatability, (iii) it has a reduced induction potential mediated by the pregnane X receptor (PXR; also known as the nuclear receptor subfamily 1, group 1, member 2 [NR1I2]), and (iv) it has decreased effects on adipocytes in vitro (21). COBI's lack of anti-HIV activity also eliminates the potential for selection of PI resistance mutations when boosting non-HIV PI drugs (21). Clinically, COBI increases systemic levels of the CYP3A substrates midazolam and elvitegravir (EVG) to a similar extent as RTV (7, 13), and coadministration with COBI allows EVG to be administered once daily (16). A fixed-dose, once-daily, single-tablet regimen that includes EVG/COBI and the nucleos(t)ide reverse transcriptase inhibitors tenofovir disoproxil fumarate and emtricitabine, colloquially known as "QUAD," has completed registrational trials, including meeting its primary endpoints in phase 3 studies (2,3,19,20).Inhibition of efflux transporters expressed in the intestine can serve as a secondary mechanism for a pharmacoenhancer to increase systemic exposure to coadministered drugs by increasing their absorption. P-glycoprotein (Pgp; also known as multidrug resistance protein 1 [MDR1] or ATP-binding cassette subfamily B member 1 [ABCB1]) and the breast cancer resistance protein (BCRP; also known as ATP-binding cassette subfamily G member 2 [ABCG2]), both expressed at the apical side of the small intestine, have been highlighted by regulatory agencies and in the literature as key transporters affecting xenobiotic pharmacokinetics (5, 6). In addition to the role of CYP3A enzymes in RTV boosting, HIV PIs are known to be substrates for transporters, including Pgp (11). Determination of the relative roles of transport and CYP3A inhibition in limiting HIV PI exposure is difficult due to their being sub...