On the Single-Crystal Structure of Tenofovir Alafenamide Mono-Fumarate: A Metastable Phase Featuring a Mixture of Co-Crystal and Salt.

Tenofovir alafenamide (TAF) is a prodrug of tenofovir as a potent nucleotide reverse transcriptase inhibitor. It serves as the key component of Genvoya® for the first-line treatment of human immunodeficiency virus infection (HIV) and is the active component of Vemlidy® for the treatment of chronic hepatitis B. Vemlidy® is also a monotherapeutic regimen formulated as TAF hemifumarate (1; TAF:fumarate = 2:1). In this work, we report for the first time the single-crystal structure of TAF fumarate hemihydrate (2, TAF:fumarate:H2O = 2:2:1). Compound 2 is initially documented as a salt in which one proton of the fumaric acid migrates to the amine group of the adenine moiety in TAF. It was recently proposed that ca. 20-30% proton is transferred to the N atom on the aromatic adenine backbone. We herein provide definitive single-crystal X-ray diffraction results to confirm that 2, though phase pure, is formed as a mixture of co-crystal (75%) and salt (25%). It features two pairs of TAF fumarates, wherein one of the four H atoms on the fumaric acid is transferred to the N atom of the adjacent adenine moiety while the other three carboxylates remain in their intrinsic acid form. Compound 2 is a metastable phase during the preparation of 1 and can be isolated by halting the reaction during the refluxing of TAF and fumaric acid in acetonitrile (MeCN). Our report complements the previous characterizations of TAF monofumarate, and its elusive structural patterns are finally deciphered.

Octadecyloxyethyl benzyl tenofovir: A novel tenofovir diester provides sustained intracellular levels of tenofovir diphosphate.

Pre-exposure prophylaxis (PrEP) with topically or systemically administered antiretroviral agents can prevent acquisition of human immunodeficiency virus type 1 (HIV-1) infection. However, in clinical trials using tenofovir-containing agents, HIV-1 acquisition is reduced but not eliminated. Incomplete adherence remains the major contributor to failure. Sustained release or long-acting antiretroviral agents may provide better HIV-1 protection by reducing the clinical impact of incomplete adherence. To reduce dosing frequency, we synthesized a novel tenofovir prodrug, octadecyloxyethyl benzyl tenofovir (ODE-Bn-TFV), that is designed to release TFV slowly in tissues, and showed potent anti-HIV activity in vitro (EC50 = 1.7 nM). In cells exposed to 14C labeled TFV, ODE-Bn-TFV or the quickly activated monoester ODE-TFV, rapid cellular uptake for both lipophilic analogs was noted, achieving 50-fold higher levels than unmodified TFV after 48 h. Following exposure to ODE-[8-14C]TFV, the intracellular diphosphate levels were approximately four-fold higher than with ODE-Bn-TFV. However, intracellular TFVpp drug levels fell rapidly yielding a half-life of about two days. TFVpp levels in ODE-Bn-TFV treated cells decreased much more slowly and reached half-maximal levels in about seven days. These results suggest early accumulation of ODE-Bn-TFV followed by sustained intracellular release following cleavage of the ester bonds linking the ODE and benzyl moieties to the active molecular precursor, thereby potentially allowing for less frequent administration than with more rapidly activated forms of tenofovir.

Next-Generation Reduction Sensitive Lipid Conjugates of Tenofovir: Antiviral Activity and Mechanism of Release.

The pharmacokinetic properties of tenofovir (TFV) and other charged nucleoside analogues are dramatically improved upon conjugation to a lipid prodrug. We previously prepared reduction-sensitive lipid conjugates of TFV that demonstrate superior antiviral activity compared to other lipid conjugates including the clinically approved formulation, tenofovir disoproxil fumarate (TDF). In continuation of that work, we have synthesized next-generation conjugates with reduced cytotoxicity that retain potent antiviral activity against HIV-1 and HBV with a therapeutic index >100000 for our most potent conjugate. We also show that disulfide reduction is not responsible for prodrug cleavage unless 3-exo-tet intramolecular cyclization can occur, suggesting that enzymatic hydrolysis is predominantly responsible for activity of our prodrugs in vitro.

Reduction Sensitive Lipid Conjugates of Tenofovir: Synthesis, Stability, and Antiviral Activity.

The therapeutic value of numerous small molecules hinges on their ability to permeate the plasma membrane. This is particularly true for tenofovir (TFV), adefovir, and other antiviral nucleosides that demonstrate potent antiviral activity but poor bioavailability. Using TFV as a model substrate, we hybridized two disparate prodrug strategies to afford novel reduction-sensitive lipid conjugates of TFV that exhibit subnanomolar activity toward HIV-1 and are stable in human plasma for more than 24 h with a therapeutic index approaching 30000. These compounds significantly rival the clinically approved formulation of TFV and revitalize the potential of disulfide-bearing prodrugs which have seen limited in vitro and in vivo success since their debut over 20 years ago. We further demonstrate the utility of these conjugates as a tool to indirectly probe the enzymatic hydrolysis of phosphonomonoesters that may further advance the development of other prodrug strategies for nucleosides, peptides, and beyond.