Recently, there has been an increasing focus on the pursuit of targets considered to be less druggable that offer potential for development of promising new therapeutic agents for the treatment of diseases with large unmet medical need, particularly in the areas of oncology and virology. However, conducting drug discovery campaigns in "beyond rule of 5" (bRo5) chemical space presents a significant drug design and development challenge to medicinal chemists to achieve acceptable oral pharmacokinetics. Retrospective analysis of past successes and failures in drug discovery bRo5 may shed light on the key principles that contribute to the oral bioavailability of successful bRo5 compounds and improve the efficiency of drug design for future projects. We present here highlights and case studies of lessons learned from discovery of bRo5 compounds. A simple multiparametric scoring function (AB-MPS) was devised that correlated preclinical PK results with cLogD, number of rotatable bonds, and number of aromatic rings.

Ombitasvir (ABT-267) is a hepatitis C virus (HCV) NS5A inhibitor with picomolar potency, pan-genotypic activity, and 50% effective concentrations (EC 50 s) of 0.82 to 19.3 pM against HCV genotypes 1 to 5 and 366 pM against genotype 6a. Ombitasvir retained these levels of potency against a panel of 69 genotype 1 to 6 chimeric replicons containing the NS5A gene derived from HCV-infected patients, despite the existence of natural sequence diversity within NS5A. In vitro resistance selection identified variants that conferred resistance to ombitasvir in the HCV NS5A gene at amino acid positions 28, 30, 31, 58, and 93 in genotypes 1 to 6. Ombitasvir was evaluated in vivo in a 3-day monotherapy study in 12 HCV genotype 1-infected patients at 5, 25, 50, or 200 mg dosed once daily. All patients in the study were HCV genotype 1a infected and were without preexisting resistant variants at baseline as determined by clonal sequencing. Decreases in HCV RNA up to 3.1 log 10 IU/ml were observed. Resistance-associated variants at position 28, 30, or 93 in NS5A were detected in patient samples 48 hours after the first dose. Clonal sequencing analysis indicated that wild-type virus was largely suppressed by ombitasvir during 3-day monotherapy, and at doses higher than 5 mg, resistant variant M28V was also suppressed. Ombitasvir was well tolerated at all doses, and there were no serious or severe adverse events. HCV genotype 1, predominant in North America, Europe, and Japan, accounts for 60% of the global infections (4-6). Genotype 2 infections are most prevalent in North America, Europe, and Japan, while genotype 3, 6, and 7 infections are predominant within various parts of Southeast Asia (3, 7-9). In Egypt, HCV infections are almost exclusively genotype 4, while genotype 5 is common in South Africa (10, 11). The levels of nucleotide sequence diversity between genotypes and between subtypes are 30 to 35% and 20 to 25%, respectively (12). The viral dynamics are rapid for HCV, with 10 12 virions being produced daily with a half-life of 45 min (13). Moreover, the RNA-dependent RNA polymerase of HCV is intrinsically error prone, and its lack of a proofreading function allows for introduction of approximately one nucleotide change per genome per replication cycle, which under drug pressure results in the expansion of preexisting drug resistant variants (13). These factors have created challenges in developing pan-genotypic HCV inhibitors with high genetic barriers to the development of resistance.HCV replication can be inhibited at various points in the replication cycle by targeting viral or host cell functions (14,15). For the treatment of HCV genotype 1, three HCV NS3/4A protease inhibitors (telaprevir, boceprevir, and simeprevir) and one nucleoside NS5B polymerase inhibitor (sofosbuvir), each in combination with pegylated interferon (pegIFN) and ribavirin (RBV), have received marketing approval in the United States and Europe. The sustained virologic response (SVR) rate increased from 40 to 52% with pegIFN and RBV regimens to 67...

We describe here N-phenylpyrrolidine-based inhibitors of HCV NS5A with excellent potency, metabolic stability, and pharmacokinetics. Compounds with 2S,5S stereochemistry at the pyrrolidine ring provided improved genotype 1 (GT1) potency compared to the 2R,5R analogues. Furthermore, the attachment of substituents at the 4-position of the central N-phenyl group resulted in compounds with improved potency. Substitution with tert-butyl, as in compound 38 (ABT-267), provided compounds with low-picomolar EC50 values and superior pharmacokinetics. It was discovered that compound 38 was a pan-genotypic HCV inhibitor, with an EC50 range of 1.7-19.3 pM against GT1a, -1b, -2a, -2b, -3a, -4a, and -5a and 366 pM against GT6a. Compound 38 decreased HCV RNA up to 3.10 log10 IU/mL during 3-day monotherapy in treatment-naive HCV GT1-infected subjects and is currently in phase 3 clinical trials in combination with an NS3 protease inhibitor with ritonavir (r) (ABT-450/r) and an NS5B non-nucleoside polymerase inhibitor (ABT-333), with and without ribavirin.

There are currently (July, 2002) six protease inhibitors approved for the treatment of HIV infection, each of which can be classified as peptidomimetic in structure. These agents, when used in combination with other antiretroviral agents, produce a sustained decrease in viral load, often to levels below the limits of quantifiable detection, and a significant reconstitution of the immune system. Therapeutic regimens containing one or more HIV protease inhibitors thus provide a highly effective method for disease management. The important role of protease inhibitors in HIV therapy, combined with numerous challenges remaining in HIV treatment, have resulted in a continued effort both to optimize regimens using the existing agents and to identify new protease inhibitors that may provide unique properties. This review will provide an overview of the discovery and clinical trials of the currently approved HIV protease inhibitors, followed by an examination of important aspects of therapy, such as pharmacokinetic enhancement, resistance and side effects. A description of new peptidomimetic compounds currently being investigated in the clinic and in preclinical discovery will follow.

Direct-acting antiviral regimens have transformed therapeutic management of hepatitis C across all prevalent genotypes. Most of the chemical matter in these regimens comprises molecules well outside the traditional drug development chemical space and presents significant challenges. Herein, the implications of high conformational flexibility and the presence of a 15-membered macrocyclic ring in paritaprevir are studied through a combination of advanced computational and experimental methods with focus on molecular chameleonicity and crystal form complexity. The ability of the molecule to toggle between high and low 3D polar surface area (PSA) conformations is underpinned by intramolecular hydrogen bonding (IMHB) interactions and intramolecular steric effects. Computational studies consequently show a very significant difference of over 75 Å 2 in 3D PSA between polar and apolar environments and provide the structural basis for the perplexingly favorable passive permeability of the molecule. Crystal packing and protein binding resulting in strong intermolecular interactions disrupt these intramolecular interactions. Crystalline Form I benefits from strong intermolecular interactions, whereas the weaker intermolecular interactions in Form II are partially compensated by the energetic advantage of an IMHB. Like Form I, no IMHB is observed within the receptor-bound conformation; instead, an intermolecular H-bond contributes to the potency of the molecule. The choice of metastable Form II is derisked through strategies accounting for crystal surface and packing features to manage higher form specific solid-state chemical reactivity and specific processing requirements. Overall, the results show an unambiguous link between structural features and derived properties from crystallization to dissolution, permeation, and docking into the protein pocket.

Drug discovery efforts at Abbott Laboratories have led to the identification of influenza neuraminidase inhibitor A-315675 (1) as a candidate for development as an antiinfluenza drug. A convergent, stereoselective synthesis of this highly functionalized pyrrolidine is reported that utilizes pyrrolinone 2 as the key intermediate. The C5, C6 stereochemistry was established through a diastereoselective condensation of chiral imine compound 3 with silyloxypyrrole 4 to give pyrrolinone 2. The stereochemical outcome of this reaction depended critically on the choice of the imine functional group (FG), with tritylsulfenyl and (R)-toluenesulfinyl providing the desired products in good yields as crystalline intermediates. Conversion of pyrrolinone 2 into 1 was accomplished in seven subsequent steps, including Michael addition of cis-1-propenylcuprate at C4 and introduction of a cyano group as a carboxylic acid equivalent at C2.

We studied the synthesis, cleavage rates, and oral administration of prodrugs of the HIV protease inhibitors (PIs) lopinavir and ritonavir. Phosphate esters attached directly to the central hydroxyl groups of these PIs did not demonstrate enzyme-mediated cleavage in vitro and did not provide measurable plasma levels of the parent drugs in vivo. However, oxymethylphosphate (OMP) and oxyethylphosphate (OEP) prodrugs provided improved rates of cleavage, high levels of aqueous solubility, and high plasma levels of the parent drugs when dosed orally in rats and dogs. Dosing unformulated capsules containing the solid prodrugs led to plasma levels equivalent to those observed for dosing formulated solutions of the parent drugs. A direct synthetic process for the preparation of OMP and OEP prodrugs was developed, and the improved synthetic method may be applicable to the preparation of analogous soluble prodrugs of other drug classes with limited solubility.

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