Duncan Robert Armour

Maraviroc (UK-427,857), a Potent, Orally Bioavailable, and Selective Small-Molecule Inhibitor of Chemokine Receptor CCR5 with Broad-Spectrum Anti-Human Immunodeficiency Virus Type 1 Activity

ABSTRACT Maraviroc (UK-427,857) is a selective CCR5 antagonist with potent anti-human immunodeficiency virus type 1 (HIV-1) activity and favorable pharmacological properties. Maraviroc is the product of a medicinal chemistry effort initiated following identification of an imidazopyridine CCR5 ligand from a high-throughput screen of the Pfizer compound file. Maraviroc demonstrated potent antiviral activity against all CCR5-tropic HIV-1 viruses tested, including 43 primary isolates from various clades and diverse geographic origin (geometric mean 90% inhibitory concentration of 2.0 nM). Maraviroc was active against 200 clinically derived HIV-1 envelope-recombinant pseudoviruses, 100 of which were derived from viruses resistant to existing drug classes. There was little difference in the sensitivity of the 200 viruses to maraviroc, as illustrated by the biological cutoff in this assay (= geometric mean plus two standard deviations [SD] of 1.7-fold). The mechanism of action of maraviroc was established using cell-based assays, where it blocked binding of viral envelope, gp120, to CCR5 to prevent the membrane fusion events necessary for viral entry. Maraviroc did not affect CCR5 cell surface levels or associated intracellular signaling, confirming it as a functional antagonist of CCR5. Maraviroc has no detectable in vitro cytotoxicity and is highly selective for CCR5, as confirmed against a wide range of receptors and enzymes, including the hERG ion channel (50% inhibitory concentration, >10 μM), indicating potential for an excellent clinical safety profile. Studies in preclinical in vitro and in vivo models predicted maraviroc to have human pharmacokinetics consistent with once- or twice-daily dosing following oral administration. Clinical trials are ongoing to further investigate the potential of using maraviroc for the treatment of HIV-1 infection and AIDS.

The Discovery of the CCR5 Receptor Antagonist, UK-427,857, A New Agent for the Treatment of HIV Infection and AIDS

Publisher Summary The CCR5 receptor is a member of the family of G-protein coupled receptors and is predicted to have a typical seven transmembrane structure. This class of proteins is heavily represented in the druggable genome, which gives some encouragement that a drug-like ligand should be identifiable. This chapter describes the drug discovery program that led to the identification of UK-427,857, a prototype CCR5 antagonist with excellent potency against lab-adapted and primary HIV-1 isolates, as a clinical candidate for the treatment of HIV. UK-427,857 (54) possessed excellent antiviral potency and reasonable microsomal stability combined with good selectivity over potential ion channel effects. The potency of UK-427,857 (54) was encouraging when tested against HIV BaL in peripheral blood monocytes where the receptor is expressed under its native conformations (IC 90 = 6 nM) but was even more exciting when extensively tested against primary origin HIV isolates. The chapter also deals with the strategies for minimizing cardiac toxicity, while maintaining ADME properties commensurate with low dose.

The Discovery of CCR5 Receptor Antagonists for the Treatment of HIV Infection: Hit-to-Lead Studies

Infection with HIV leads, in the vast majority of cases, to progressive disease and ultimately death. By 2004, just 23 years after AIDS was first recognised, the Joint United Nations Programme on HIV/AIDS estimated that 42 million people worldwide were infected with HIV, with more than 20 million dead since the beginning of the epidemic. Furthermore, rates of infection are once again on the increase in the developed world. Despite the undoubted achievements of highly active antiretroviral therapy (HAART) using cocktails of reverse transcriptase and protease inhibitors, there is still a high unmet medical need for better tolerated, conveniently administered agents to treat HIV and AIDS. New mechanisms of action are particularly attractive to avoid issues of viral resistance. HIV enters the host cell by fusing the lipid membrane of the virus with the host cell membrane. This fusion is triggered by the interaction of proteins on the surface of the HIV envelope with specific cell surface receptors. One of these is CD4, the main receptor for HIV-1 that binds to gp120, a surface protein on the virus particle. However, CD4 alone is not sufficient to permit HIV fusion and cell entry, an additional coreceptor from the chemokine family of G-protein coupled receptors (GPCRs) is required. The chemokine receptor CCR5 has been demonstrated to be the major coreceptor for the fusion and entry of macrophage tropic (R5-tropic) HIV-1 into cells. R5-tropic strains are prevalent in the early asymptomatic stages of infection. Indeed, the CCR5 monoclonal antibody PRO140 has been demonstrated to potently inhibit a broad range of HIV-1 strains from infecting their target cells. Shifts in tropism do occur during progression, mainly to X4 viruses that use CXCR4 as coreceptor, however, approximately 50% of individuals are infected with strains that maintain their requirement for CCR5. Currently the cause of the switch in tropism in late stage disease is unknown. There is evidence that homozygotes possessing a 32 base pair deletion in the CCR5 coding region are resistant to infection with R5-tropic HIV-1. These homozygotes do not express functional CCR5 receptors on the cell surface. Individuals who are heterozygous for the 32 base pair deletion display significantly longer progression times to the symptomatic stages of infection and evidence is emerging that they respond better to HAART. Moreover, CCR5 deficient individuals are apparently fully immunocompetent, indicating that absence of CCR5 function may not be detrimental and that a CCR5 antagonist should be well tolerated. Currently, some clinical trials involving CCR5 ligands are being halted increasing the focus on this promising mechanism for the treatment of HIV. In particular, aplaviroc was halted due to liver toxicity developed in a Phase IIb clinical trial that was also observed in a subsequent Phase III trial. Herein, the discovery of hits, design and synthesis, structure–activity relationships (SAR), and biological evaluation of UK-374,503 (1) and related compounds are described.

Overcoming hERG Affinity in the Discovery of Maraviroc; A CCR5 Antagonist for the Treatment of HIV

Avoiding cardiac liability associated with blockade of hERG (human ether a go-go) is key for successful drug discovery and development. This paper describes the work undertaken in the discovery of a potent CCR5 antagonist, maraviroc 34, for the treatment of HIV. In particular the use of a pharmacophore model of the hERG channel and a high throughput binding assay for the hERG channel are described that were critical to elucidate SAR to overcome hERG liabilities. The key SAR involves the introduction of polar substituents into regions of the molecule where it is postulated to undergo hydrophobic interactions with the ion channel. Within the CCR5 project there appeared to be no strong correlation between hERG affinity and physiochemical parameters such as pKa or lipophilicity. It is believed that chemists could apply these same strategies early in drug discovery to remove hERG interactions associated with lead compounds while retaining potency at the primary target.

The Discovery of Tropane‐derived CCR5 Receptor Antagonists

The development of compound 1, a piperidine‐based CCR5 receptor antagonist with Type I CYP2D6 inhibition, into the tropane‐derived analogue 5, is described. This compound, which is devoid of CYP2D6 liabilities, is a highly potent ligand for the CCR5 receptor and has broad‐spectrum activity against a range of clinically relevant HIV isolates. The identification of human ether a‐go‐go‐related gene channel inhibition within this series is described and the potential for QTc interval prolongation discussed. Furthermore, structure activity relationship (SAR) around the piperidine moiety is also described.

Novel silyl linkers for solid-phase synthesis

The syntheses of two silyl chloride resins, 2 and 10 are described starting from Merrifield resin, 3-methyl-1,3-butanediol and diphenyldichlorosilane or dimethyldichlorosilane, respectively. The silyl chloride resin 2 was used for the attachment of 1° alcohols, 2° alcohols and phenols to the solid phase. A preliminary study of the stability of the diphenylsiloxane linker towards certain reaction conditions was also carried out. A more reactive silyl chloride resin 10 was found to be suitable for the attachment of 3° alcohols to the solid phase.