This chemistry paved way for multi
This chemistry paved way for multi-gram synthesis of which was utilized for extensive piperazine benzamide SAR exploration. Depicted in , the two-step operation towards final targets involved standard amide coupling conditions with EDC or HATU followed by Boc deprotection under acidic conditions. Some of the SAR in the piperazine series was developed in conjunction with non-piperazine benzamides that were being investigated at the time. As previously mentioned, one of our initial objectives was to improve PK while maintaining potency and selectivity. Earlier disclosures from our laboratories describe the importance of reducing risk for inducing phospholipidosis (PLDS) and poor selectivities over the serotonin transporter (SERT)., In order to identify any potential cardiac liabilities the analogs were also screened against the human Ether-a-go-go Related Gene (hERG) potassium channel and the human cardiac sodium channel (hNa1.5). The group reasoned that with the shorter CN bond length associated with piperazine in comparison to piperidine it might be necessary to homologate substituents by one carbon in order to emulate β-AR activity observed in the piperidine class. Initiated with these objectives in mind, one of the first compounds synthesized was a relatively simple homologated 2-pyridyl piperazine . This PD128907 HCl proved to be an excellent starting point in the series. Although its β-AR potency (EC=20nM) was about 7 times less potent than , the compound exhibited excellent selectivities over binding to β-ARs, cardiac ion channels, SERT and also showed minimal risk for phospholipidosis (=54μM). The preclinical pharmacokinetic results for are shown in and illustrate high unbound plasma clearance (148mL/min/kg) and a moderate half-life (t=2.6h) in rat. In addition, its profile was improved in dog and rhesus exhibiting much lower unbound clearances and increased half-lives in dog t (12h) and rhesus (5.8h). The rest of the compounds highlighted in this report center around ester bioisosteres which also proved beneficial in the non-piperazine series with enhanced β-AR potency and improvements in PK. In general, the relevant off-target activities continued to be well maintained throughout the series (). Utilizing a similar oxadiazole substituent as , a significant improvement in β-AR potency was observed with (EC=2.5nM). Compound , the 1,2,4-oxadiazole regioisomer of , further improved β-AR potency into the sub-nanomolar range (EC=0.32nM). Unfortunately, suffered from suboptimal rat PK (). Difluoro ( and trifluoro ( ethyl substituted triazoles were also well tolerated with β-AR EC of 2.9nM and 1.6nM respectively. Despite having longer half-lives in rat, these triazoles exhibited high clearance (⩾105mL/min/kg) and low oral bioavailabilities (<1%) and were not further developed. A group of substituted tetrazoles was also investigated resulting in a slight loss in β-AR activity observed with aliphatic substituents, (EC=5.1nM) and (EC=5.2) compared to their difluoro- and trifluoroethyl counterparts, (EC=1.5nM) and (EC=1.9nM) respectively. As observed with , compounds and showed PK improvements in dog relative to rat, highlighted by lower unbound clearance and longer half-lives. The clearance of in dog was low (22mL/min/kg) with a long half-life (t=8.5h). Dog unbound exposure and bioavailability of were modest (PO AUC=0.19μM·h/mpk and F=20%). Trifluoroethyl tetrazole showed further improvement in dog PK with higher unbound exposure (PO AUC=1.2μM·h/mpk) and bioavailability (F=100%) in conjunction with a relatively long half-life (t=6.9h). Unfortunately, however, also exhibited poor oral exposure and bioavailability in rhesus. Evaluation of in cross-species liver microsome incubations at least partially explains this trend, including the sub-optimal rhesus PK observed, with the order of stability being dog>rat>human>rhesus (% parent @ 45min=85% dog, 71% rat, 42% human, 19% rhesus). In addition, compound proved stable up to 60min hepatocyte incubations in human (92% parent remaining) and dog (100%) with moderate stabilities in rat (73%) and rhesus (50%). As a result, was deemed a lead compound in the series with an overall preclinical pharmacokinetic profile projected to be suitable for QD dosing in humans.