Development of Highly Optimized Antibody-Drug Conjugates against CD33 and CD123 for Acute Myeloid Leukemia.

PURPOSE: Mortality due to acute myeloid leukemia (AML) remains high, and the management of relapsed or refractory AML continues to be therapeutically challenging. The reapproval of Mylotarg, an anti-CD33-calicheamicin antibody-drug conjugate (ADC), has provided a proof of concept for an ADC-based therapeutic for AML. Several other ADCs have since entered clinical development of AML, but have met with limited success. We sought to develop a next-generation ADC for AML with a wide therapeutic index (TI) that overcomes the shortcomings of previous generations of ADCs. EXPERIMENTAL DESIGN: We compared the TI of our novel CD33-targeted ADC platform with other currently available CD33-targeted ADCs in preclinical models of AML. Next, using this next-generation ADC platform, we performed a head-to-head comparison of two attractive AML antigens, CD33 and CD123. RESULTS: Our novel ADC platform offered improved safety and TI when compared with certain currently available ADC platforms in preclinical models of AML. Differentiation between the CD33- and CD123-targeted ADCs was observed in safety studies conducted in cynomolgus monkeys. The CD33-targeted ADC produced severe hematologic toxicity, whereas minimal hematologic toxicity was observed with the CD123-targeted ADC at the same doses and exposures. The improved toxicity profile of an ADC targeting CD123 over CD33 was consistent with the more restricted expression of CD123 in normal tissues. CONCLUSIONS: We optimized all components of ADC design (i.e., leukemia antigen, antibody, and linker-payload) to develop an ADC that has the potential to translate into an effective new therapy against AML.

Enhanced Antitumor Activity of an Anti-5T4 Antibody-Drug Conjugate in Combination with PI3K/mTOR inhibitors or Taxanes.

PURPOSE: Targeted treatment of solid or liquid tumors with antibody-drug conjugates (ADCs) can lead to promising clinical benefit. The aim of the study is to investigate combination regimens of auristatin-based ADCs in preclinical models of cancer. EXPERIMENTAL DESIGN: An auristatin-based anti-5T4 antibody conjugate (5T4-ADC) and auristatin payloads were combined with the dual PI3K/mTOR catalytic site inhibitor PF-05212384 (PF-384) or taxanes in a panel of tumor cell lines. Drug interactions in vitro were evaluated using cell viability assays, apoptosis induction, immunofluorescence, mitotic index, and immunoblotting. Breast cancer cells treated with auristatin analogue or 5T4-ADC were profiled by total- and phospho-proteomics. Antitumor efficacy of selected combinations was evaluated in 5T4-positive human breast or lung tumor xenografts in vivo. RESULTS: In vitro, auristatin-based agents displayed strong synergistic or additive activity when combined with PF-384 or taxanes, respectively. Further, treatment of 5T4-ADC plus PF-384 resulted in stronger induction of apoptosis and cell line-specific attenuation of pAKT and pGSK. Interestingly, proteomic analysis revealed unique effects of auristatins on multiple components of mRNA translation. Addition of PF-384 further amplified effects of 5T4-ADC on translational components, providing a potential mechanism of synergy between these drugs. In human tumor xenografts, dual targeting with 5T4-ADC/PF-384 or 5T4-ADC/paclitaxel produced substantially greater antitumor effects with longer average survival as compared with monotherapy treatments. CONCLUSIONS: Our results provide a biologic rationale for combining 5T4-ADC with either PI3K/mTOR pathway inhibitors or taxanes and suggest that mechanisms underlying the synergy may be attributed to cellular effects of the auristatin payload.

Discovery of 7-aminofuro[2,3-c]pyridine inhibitors of TAK1: optimization of kinase selectivity and pharmacokinetics.

The kinase selectivity and pharmacokinetic optimization of a series of 7-aminofuro[2,3-c]pyridine inhibitors of TAK1 is described. The intersection of insights from molecular modeling, computational prediction of metabolic sites, and in vitro metabolite identification studies resulted in a simple and unique solution to both of these problems. These efforts culminated in the discovery of compound 13a, a potent, relatively selective inhibitor of TAK1 with good pharmacokinetic properties in mice, which was active in an in vivo model of ovarian cancer.

Novel 6-aminofuro[3,2-c]pyridines as potent, orally efficacious inhibitors of cMET and RON kinases.

A series of novel 6-aminofuro[3,2-c]pyridines as kinase inhibitors is described, most notably, OSI-296 (6). We discuss our exploration of structure-activity relationships and optimization leading to OSI-296 and disclose its pharmacological activity against cMET and RON in cellular assays. OSI-296 is a potent and selective inhibitor of cMET and RON kinases that shows in vivo efficacy in tumor xenografts models upon oral dosing and is well tolerated.

Discovery of novel insulin-like growth factor-1 receptor inhibitors with unique time-dependent binding kinetics.

This letter describes a series of small molecule inhibitors of IGF-1R with unique time-dependent binding kinetics and slow off-rates. Structure-activity and structure-kinetic relationships were elucidated and guided further optimizations within the series, culminating in compound 2. With an IGF-1R dissociative half-life (t 1/2) of >100 h, compound 2 demonstrated significant and extended PD effects in conjunction with tumor growth inhibition in xenograft models at a remarkably low and intermittent dose, which correlated with the observed in vitro slow off-rate properties.

Preclinical characterization of OSI-027, a potent and selective inhibitor of mTORC1 and mTORC2: distinct from rapamycin.

The phosphoinositide 3-kinase (PI3K)/AKT/mTOR pathway is frequently activated in human cancers, and mTOR is a clinically validated target. mTOR forms two distinct multiprotein complexes, mTORC1 and mTORC2, which regulate cell growth, metabolism, proliferation, and survival. Rapamycin and its analogues partially inhibit mTOR through allosteric binding to mTORC1, but not mTORC2, and have shown clinical utility in certain cancers. Here, we report the preclinical characterization of OSI-027, a selective and potent dual inhibitor of mTORC1 and mTORC2 with biochemical IC(50) values of 22 nmol/L and 65 nmol/L, respectively. OSI-027 shows more than 100-fold selectivity for mTOR relative to PI3Kα, PI3Kß, PI3Kγ, and DNA-PK. OSI-027 inhibits phosphorylation of the mTORC1 substrates 4E-BP1 and S6K1 as well as the mTORC2 substrate AKT in diverse cancer models in vitro and in vivo. OSI-027 and OXA-01 (close analogue of OSI-027) potently inhibit proliferation of several rapamycin-sensitive and -insensitive nonengineered and engineered cancer cell lines and also, induce cell death in tumor cell lines with activated PI3K-AKT signaling. OSI-027 shows concentration-dependent pharmacodynamic effects on phosphorylation of 4E-BP1 and AKT in tumor tissue with resulting tumor growth inhibition. OSI-027 shows robust antitumor activity in several different human xenograft models representing various histologies. Furthermore, in COLO 205 and GEO colon cancer xenograft models, OSI-027 shows superior efficacy compared with rapamycin. Our results further support the important role of mTOR as a driver of tumor growth and establish OSI-027 as a potent anticancer agent. OSI-027 is currently in phase I clinical trials in cancer patients.

Imidazo[1,5-a]pyrazines: orally efficacious inhibitors of mTORC1 and mTORC2.

The discovery and optimization of a series of imidazo[1,5-a]pyrazine inhibitors of mTOR is described. HTS hits were optimized for potency, selectivity and metabolic stability to provide the orally bioavailable proof of concept compound 4c that demonstrated target inhibition in vivo and concomitant inhibition of tumor growth in an MDA-MB-231 xenograft model.

High-throughput screening for mTORC1/mTORC2 kinase inhibitors using a chemiluminescence-based ELISA assay.

A high-throughput chemiluminescence and ELISA-based biochemical assay to identify mTORC1/mTORC2 kinase inhibitors is described. These mTOR complexes were isolated from HeLa whole cell lysate using mTOR antibodies and in-well immunoprecipitation. The integrity and purity of the mTORC1 and mTORC2 immunocomplexes were confirmed by western blotting. Full-length recombinant 4E-BP1 was used as a substrate and the catalytic activity was measured by detection of p4E-BP1 [T37/46] by a chemiluminescence method. The performance of this assay that can be used to identify dual mTORC1 and mTORC2 kinase inhibitors in a high-throughput 384-well format is described.

Kinetic analysis of epidermal growth factor receptor somatic mutant proteins shows increased sensitivity to the epidermal growth factor receptor tyrosine kinase inhibitor, erlotinib.

We show that two commonly occurring epidermal growth factor receptor (EGFR) somatic mutations, L858R and an in-frame deletion mutant, Del(746-750), exhibit distinct enzymatic properties relative to wild-type EGFR and are differentially sensitive to erlotinib. Kinetic analysis of the purified intracellular domains of EGFR L858R and EGFR Del(746-750) reveals that both mutants are active but exhibit a higher K(M) for ATP and a lower K(i) for erlotinib relative to wild-type receptor. When expressed in NR6 cells, a cell line that does not express EGFR or other ErbB receptors, both mutations are ligand dependent for receptor activation, can activate downstream EGFR signaling pathways, and promote cell cycle progression. As expected from the kinetic analysis, the EGFR Del(746-752) is more sensitive to erlotinib inhibition than the EGFR L858R mutant. Further characterization shows that these mutations promote ligand-dependent and anchorage-independent growth, and cells harboring these mutant receptors form tumors in immunocompromised mice. Analysis of tumor lysates reveals that the tumorigenicity of the mutant EGFR cell lines may be due to a differential pattern of mutant EGFR autophosphorylation as compared with wild-type receptor. Significant inhibition of tumor growth, in mice harboring wild-type EGFR receptors, is only observed at doses of erlotinib approaching the maximum tolerated dose for the mouse. In contrast, the growth of mutant tumors is inhibited by erlotinib treatment at approximately one third the maximum tolerated dose. These findings suggest that EGFR somatic mutations directly influence both erlotinib sensitivity and cellular transformation.

OSI-930: a novel selective inhibitor of Kit and kinase insert domain receptor tyrosine kinases with antitumor activity in mouse xenograft models.

OSI-930 is a novel inhibitor of the receptor tyrosine kinases Kit and kinase insert domain receptor (KDR), which is currently being evaluated in clinical studies. OSI-930 selectively inhibits Kit and KDR with similar potency in intact cells and also inhibits these targets in vivo following oral dosing. We have investigated the relationships between the potency observed in cell-based assays in vitro, the plasma exposure levels achieved following oral dosing, the time course of target inhibition in vivo, and antitumor activity of OSI-930 in tumor xenograft models. In the mutant Kit-expressing HMC-1 xenograft model, prolonged inhibition of Kit was achieved at oral doses between 10 and 50 mg/kg and this dose range was associated with antitumor activity. Similarly, prolonged inhibition of wild-type Kit in the NCI-H526 xenograft model was observed at oral doses of 100 to 200 mg/kg, which was the dose level associated with significant antitumor activity in this model as well as in the majority of other xenograft models tested. The data suggest that antitumor activity of OSI-930 in mouse xenograft models is observed at dose levels that maintain a significant level of inhibition of the molecular targets of OSI-930 for a prolonged period. Furthermore, pharmacokinetic evaluation of the plasma exposure levels of OSI-930 at these effective dose levels provides an estimate of the target plasma concentrations that may be required to achieve prolonged inhibition of Kit and KDR in humans and which would therefore be expected to yield a therapeutic benefit in future clinical evaluations of OSI-930.