Compounds and were evaluated for their pharmacokinetic prope

Compounds and were evaluated for their pharmacokinetic properties in rats and the results are summarized in . Following intravenous (IV) administration, both compounds displayed low clearance (CL), short half-lives (t) and low volumes of distribution (V). When dosed orally as a solution, exposure was high and bioavailability (F) was >65% for both compounds. It was reasoned that the subtle differences in the antiviral activity of the saturated and unsaturated matched pairs and the lower serum shift for may be, in part, a function of the geometrical relationship between the pyrimidinone core and the C2-substituent. In the case of the unsaturated substituent, it was hypothesized that the sp hybridization of the connecting carbon would favor a coplanar arrangement between the pyrimidinone and the C2 substituent. In contrast, the tetrahedral geometry (sp hybridization) associated with the connecting carbon 1400w in the saturated analogs would theoretically favor a topography in which the pyrimidinone ring is perpendicular to the plane of the 2 substituent. In support of this hypothesis, the single crystal X-ray structure of revealed the pyrimidinone core to be perpendicular to the plane of the tetrahydrofuranyl ring (). With this observation in mind, the next phase of the SAR survey focused upon designing compounds that would lock the geometric relationship between the pyrimidinone core and the C2-substituent. To that end, it was envisioned that this could be accomplished by linking the pyrimidinone N1 to the 3′-carbon atom of the tetrahydrofuran ring to establish a rigid framework on which to deploy a spirocycle, as depicted conceptually by in . In this arrangement, the pyrimidinone core is perpendicular to the plane of the spirocyclic ring, thereby mimicking the topography observed in the solid state structure of . This design concept was examined initially in the context of the spirocyclopentane derivative for which the single crystal X-ray confirmed the design hypothesis (). The , antiviral activity and rat clearance of the series of spirocyclic benzyl carboxamides – are compiled in . Compound displayed superior antiviral potency compared to the progenitor and, interestingly, displayed a lower serum shift despite being more lipophilic than . Benzylamide , containing a lipophilic -methyl substituent, maintained both the intrinsic enzyme inhibition and antiviral activity of , but suffered from a substantial (30-fold) loss of activity in the presence of HSA. In an effort to reduce the effect of serum, the benzylamides series incorporating hydrophilic substituents were surveyed. The methylsulfonyl-substituted analog displayed potent antiviral activity with a modest 8-fold shift in the EC value in the presence of HSA, whereas the dimethylsulfamoyl analog exhibited slightly reduced activity in both the strand transfer and cell culture inhibition assays compared to with or without added human serum albumen. On the other hand, the morpholinosulfonamide analog exhibited reduced potency in the biochemical assay but demonstrated a similar antiviral profile to in both cell culture assays. Lastly, the cyclic sulfonamide exhibited a comparable antiviral profile to in all three assays. Of the spirocyclic carboxamide derivatives that were evaluated in a rat IV pharmacokinetic screen, and displayed intermediate clearance values, while exhibited high clearance. Although the spirocyclic derivatives demonstrated targeted antiviral potency, these compounds still suffered from poor pharmacokinetic and pharmaceutical/physicochemical properties. These compounds displayed aqueous solubility of less ≤5 μg/ml, providing a focus for further optimization that was also anticipated to offer the potential for enhanced pharmacokinetic properties. To reduce the lipophilicity of , the effect of replacing the cyclopentyl ring with a -dimethyl moiety was explored, while the introduction of a heteroatom into the fused B ring was examined as an avenue toward further increasing polarity (). The initial efforts were directed at scaffold which incorporates an embedded morpholine heterocycle, a moiety that has found extensive application as a means of improving physicochemical properties. Analysis of developability data for compounds containing a variety of heterocyclic rings has found that those containing a morpholine heterocycle fared well when compared to other heterocycles, generally displaying improved aqueous solubility, lower protein binding and a lower potential for CYP450 inhibition. This scaffold was anticipated to exhibit reduced lipophilic character as reflected in the LogP value of 1.02 calculated for which is substantially lower than the LogP of 2.49 calculated for .