Viability of primary osteoblasts after treatment with tenofovir alafenamide: Lack of cytotoxicity at clinically relevant drug concentrations

The Nucleoside/Nucleotide Analogs Tenofovir and Emtricitabine Are Inactive against SARS-CoV-2

The urgent response to the COVID-19 pandemic required accelerated evaluation of many approved drugs as potential antiviral agents against the causative pathogen, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Using cell-based, biochemical, and modeling approaches, we studied the approved HIV-1 nucleoside/tide reverse transcriptase inhibitors (NRTIs) tenofovir (TFV) and emtricitabine (FTC), as well as prodrugs tenofovir alafenamide (TAF) and tenofovir disoproxilfumarate (TDF) for their antiviral effect against SARS-CoV-2. A comprehensive set of in vitro data indicates that TFV, TAF, TDF, and FTC are inactive against SARS-CoV-2. None of the NRTIs showed antiviral activity in SARS-CoV-2 infected A549-hACE2 cells or in primary normal human lung bronchial epithelial (NHBE) cells at concentrations up to 50 µM TAF, TDF, FTC, or 500 µM TFV. These results are corroborated by the low incorporation efficiency of respective NTP analogs by the SARS-CoV-2 RNA-dependent-RNA polymerase (RdRp), and lack of the RdRp inhibition. Structural modeling further demonstrated poor fitting of these NRTI active metabolites at the SARS-CoV-2 RdRp active site. Our data indicate that the HIV-1 NRTIs are unlikely direct-antivirals against SARS-CoV-2, and clinicians and researchers should exercise caution when exploring ideas of using these and other NRTIs to treat or prevent COVID-19.

Forgiveness of INSTI-Containing Regimens at Drug Concentrations Simulating Variable Adherence In Vitro

The integrase strand transfer inhibitor (INSTI)-based regimens bictegravir/emtricitabine/tenofovir alafenamide (BIC/FTC/TAF), dolutegravir (DTG)+FTC/TAF, DTG/lamivudine (3TC), and DTG/rilpivirine (RPV) are all approved for treatment of HIV-infected patients, with various limitations. Here, time to in vitro viral breakthrough (VB) and resistance barrier using simulated human drug exposures at either full or suboptimal treatment adherence to each regimen were compared. At drug concentrations corresponding to full adherence and 1 missed dose (Cmin and Cmin-1), no VB occurred with any regimen. At Cmin-2, VB occurred only with DTG+3TC, with emergent resistance to both drugs. At Cmin-3, VB occurred with all regimens: 100% of DTG+3TC cultures had VB by day 12, and <15% of BIC+FTC+TAF, DTG+FTC+TAF, and DTG+RPV cultures had VB. Emergent reverse transcriptase (RT) or integrase (IN) resistance was seen with DTG+RPV and DTG+3TC but not with BIC+FTC+TAF or DTG+FTC+TAF. At Cmin-4, 100% VB occurred with DTG+3TC and DTG+FTC+TAF by day 12, while 94% VB occurred with DTG+RPV by day 25 and only 50% VB occurred with BIC+FTC+TAF by day 35. Emergent Cmin-4 drug resistance was seen with all regimens but at differing frequencies; DTG+RPV had the most cultures with resistance. Emergent resistance was consistent with clinical observations. Overall, under high adherence conditions, no in vitro VB or resistance development occurred with these INSTI-based regimens. However, when multiple missed doses were simulated in vitro, BIC+FTC+TAF had the highest forgiveness and barrier to resistance of all tested regimens. Compared to DTG+3TC and DTG+FTC+TAF, DTG+RPV had higher forgiveness but lower resistance barrier after several simulated missed doses.

Tenofovir Modulates Semaphorin 4D Signaling and Regulates Bone Homeostasis, Which Can Be Counteracted by Dipyridamole and Adenosine A2A Receptor

Semaphorin 4D (Sema4D) is a neurotrophin that is secreted by osteoclasts and binds to its receptor PlexinB1 on osteoblasts to inhibit their differentiation and function. Adenosine A2A activation inhibits osteoclast Sema4D-mediated secretion, diminishes inflammatory osteolysis and prevents bone loss following tenofovir (one of the most used antivirals in HIV). Therefore, tenofovir might activate Sema4D signaling to alter bone turnover. Female C57Bl/6/A2AKO mice were ovariectomized and treated with saline (control), tenofovir 75 mg/Kg/day, dipyridamole 25 mg/Kg/day or a combination for 5 weeks and long bones were prepared for histology. Primary murine-induced osteoclast/osteoblast were challenged with tenofovir/dipyridamole 1 μM each, and the expression of Sema4D/PlexinB1, RhoA/ROCK/IGF1R was studied by RT-PCR, Western blot and immunostaining. In vivo tenofovir showed an increased expression of Sema4D when compared to control mice, and dipyridamole reverted the expression in an A2A-dependent manner. In vitro, tenofovir increases Sema4D expression and secretion in osteoclast precursors, and pre-treatment with dipyridamole reverted this effect. pRhoA and ROCK1 activation were increased and IRS1/IGF1R expression was diminished by tenofovir in the Vav3/ARHGAP18 mechanism in osteoblast precursors and reverted by dipyridamole in an A2A-dependent manner. This suggests that tenofovir increases bone loss by activation of Sema4D/PlexinB1 signaling, which inhibits osteoblast differentiation. Agents that increase local adenosine concentrations, such as dipyridamole, might prevent bone loss following the inhibition of this pathway.

Simulating HIV Breakthrough and Resistance Development During Variable Adherence to Antiretroviral Treatment

BACKGROUND

Barriers to lifelong HIV-1 suppression by antiretrovirals include poor adherence and drug resistance; regimens with higher tolerance to missed doses (forgiveness) would be beneficial to patients. To model short-term nonadherence, in vitro experiments monitoring viral breakthrough (VB) and resistance development were conducted.

METHODS

HIV breakthrough experiments simulated drug exposures at full adherence or suboptimal adherence to bictegravir+emtricitabine+tenofovir alafenamide (BIC+FTC+TAF) or dolutegravir + lamivudine (DTG+3TC). MT-2 cells were infected with wild-type or low frequency M184V HIV-1, exposed to drug combinations, monitored for VB, and rebound virus was deep sequenced. Drug concentrations were determined using human plasma-free adjusted clinical trough concentrations (Cmin), at simulated Cmin after missing 1 to 3 consecutive doses (Cmin - 1 or Cmin - 2, and Cmin - 3) based on drug or active metabolite half-lives.

RESULTS

Cultures infected with wild-type or low frequency M184V HIV-1 showed no VB with BIC+FTC+TAF at drug concentrations corresponding to Cmin, Cmin - 1, or Cmin - 2 but breakthrough did occur in 26 of 36 cultures at Cmin - 3, where the M184V variant emerged in one culture. Experiments using DTG + 3TC prevented most breakthrough at Cmin concentrations (9/60 had breakthrough) but showed more breakthroughs as drug concentrations decreased (up to 36/36) and variants associated with resistance to both drugs emerged in some cases.

CONCLUSIONS

These in vitro VB results suggest that the high potency, long half-lives, and antiviral synergy provided by the BIC/FTC/TAF triple therapy regimen may protect from viral rebound and resistance development after short-term lapses in drug adherence.

Tenofovir alafenamide (TAF) does not deplete mitochondrial DNA in human T-cell lines at intracellular concentrations exceeding clinically relevant drug exposures

HIV-infected patients treated with certain nucleoside reverse transcriptase inhibitors (NRTIs) have experienced adverse effects due to drug-related mitochondrial toxicity. Tenofovir alafenamide (TAF) is a novel prodrug of the NRTI tenofovir (TFV) with an improved safety profile compared to tenofovir disoproxil fumarate (TDF). Prior in vitro studies have demonstrated that the parent nucleotide TFV has no significant effects on mtDNA synthesis. This study investigated whether clinically relevant TAF and TDF exposures affect mtDNA content in human lymphocytes. First, activated or resting peripheral blood mononuclear cells (PBMCs), as well as MT-2 and Jurkat T-cell lines, were continuously treated with ddC for 10 days to establish their susceptibility to mtDNA depletion. PBMCs had low sensitivity to NRTI-mediated mtDNA depletion in vitro. In contrast, ddC treatment of rapidly dividing MT-2 and Jurkat cells resulted in a dose-dependent decrease in mtDNA. Therefore, these two T-cell lines were selected for evaluating TAF and TDF treatment effects. MT-2 and Jurkat cells were pulse-treated with TAF or TDF every 24 h for 10 days to mimic pharmacologically relevant drug exposures. Pulse treatment of cells with 3.3 μM TAF or 1.1 μM TDF for 10 days resulted in 2- to 7-fold greater steady-state intracellular TFV-diphosphate (TFV-DP) levels than those observed clinically in TAF- or TDF-treated patients. At these concentrations, no significant TAF- (106.7% and 84.1% of control; p = 0.77 and 0.12 for MT-2 and Jurkat, respectively) or TDF- (100.6% and 91.0% of control; p = 0.91 and 0.37, respectively) associated reduction in mtDNA content was observed compared with untreated control cells. This study demonstrates that, despite delivering higher intracellular levels of TFV-DP than TDF, TAF does not inhibit mtDNA synthesis in vitro at concentrations exceeding the clinically relevant intracellular drug exposures. Thus, TAF has a low potential for mitochondrial toxicity in T-cells of HIV-infected patients.