David J. Frost

ABT-888, an Orally Active Poly(ADP-Ribose) Polymerase Inhibitor that Potentiates DNA-Damaging Agents in Preclinical Tumor Models

Purpose: To evaluate the preclinical pharmacokinetics and antitumor efficacy of a novel orally bioavailable poly(ADP-ribose) polymerase (PARP) inhibitor, ABT-888. Experimental Design:In vitro potency was determined in a PARP-1 and PARP-2 enzyme assay. In vivo efficacy was evaluated in syngeneic and xenograft models in combination with temozolomide, platinums, cyclophosphamide, and ionizing radiation. Results: ABT-888 is a potent inhibitor of both PARP-1 and PARP-2 with Kis of 5.2 and 2.9 nmol/L, respectively. The compound has good oral bioavailability and crosses the blood-brain barrier. ABT-888 strongly potentiated temozolomide in the B16F10 s.c. murine melanoma model. PARP inhibition dramatically increased the efficacy of temozolomide at ABT-888 doses as low as 3.1 mg/kg/d and a maximal efficacy achieved at 25 mg/kg/d. In the 9L orthotopic rat glioma model, temozolomide alone exhibited minimal efficacy, whereas ABT-888, when combined with temozolomide, significantly slowed tumor progression. In the MX-1 breast xenograft model (BRCA1 deletion and BRCA2 mutation), ABT-888 potentiated cisplatin, carboplatin, and cyclophosphamide, causing regression of established tumors, whereas with comparable doses of cytotoxic agents alone, only modest tumor inhibition was exhibited. Finally, ABT-888 potentiated radiation (2 Gy/d × 10) in an HCT-116 colon carcinoma model. In each model, ABT-888 did not display single-agent activity. Conclusions: ABT-888 is a potent inhibitor of PARP, has good oral bioavailability, can cross the blood-brain barrier, and potentiates temozolomide, platinums, cyclophosphamide, and radiation in syngeneic and xenograft tumor models. This broad spectrum of chemopotentiation and radiopotentiation makes this compound an attractive candidate for clinical evaluation.

A farnesyltransferase inhibitor improves disease phenotypes in mice with a Hutchinson-Gilford progeria syndrome mutation

Hutchinson-Gilford progeria syndrome (HGPS) is caused by the production of a truncated prelamin A, called progerin, which is farnesylated at its carboxyl terminus. Progerin is targeted to the nuclear envelope and causes misshapen nuclei. Protein farnesyltransferase inhibitors (FTI) mislocalize progerin away from the nuclear envelope and reduce the frequency of misshapen nuclei. To determine whether an FTI would ameliorate disease phenotypes in vivo, we created gene-targeted mice with an HGPS mutation (LmnaHG/+) and then examined the effect of an FTI on disease phenotypes. LmnaHG/+ mice exhibited phenotypes similar to those in human HGPS patients, including retarded growth, reduced amounts of adipose tissue, micrognathia, osteoporosis, and osteolytic lesions in bone. Osteolytic lesions in the ribs led to spontaneous bone fractures. Treatment with an FTI increased adipose tissue mass, improved body weight curves, reduced the number of rib fractures, and improved bone mineralization and bone cortical thickness. These studies suggest that FTIs could be useful for treating humans with HGPS.

Discovery of the Poly(ADP-ribose) polymerase (PARP) inhibitor 2-[(R)-2-methylpyrrolidin-2-yl]-1H-benzimidazole-4-carboxamide (ABT-888) for the treatment of cancer.

We have developed a series of cyclic amine-containing benzimidazole carboxamide PARP inhibitors with a methyl-substituted quaternary center at the point of attachment to the benzimidazole ring system. These compounds exhibit excellent PARP enzyme potency as well as single-digit nanomolar cellular potency. These efforts led to the identification of 3a (2-[(R)-2-methylpyrrolidin-2-yl]-1H-benzimidazole-4-carboxamide, ABT-888), currently in human phase I clinical trials. Compound 3a displayed excellent potency against both the PARP-1 and PARP-2 enzymes with a K(i) of 5 nM and in a C41 whole cell assay with an EC(50) of 2 nM. In addition, 3a is aqueous soluble, orally bioavailable across multiple species, and demonstrated good in vivo efficacy in a B16F10 subcutaneous murine melanoma model in combination with temozolomide (TMZ) and in an MX-1 breast cancer xenograft model in combination with either carboplatin or cyclophosphamide.

A Small-Molecule Inhibitor of Bcl-XL Potentiates the Activity of Cytotoxic Drugs In vitro and In vivo

Inhibition of the prosurvival members of the Bcl-2 family of proteins represents an attractive strategy for the treatment of cancer. We have previously reported the activity of ABT-737, a potent inhibitor of Bcl-2, Bcl-X(L), and Bcl-w, which exhibits monotherapy efficacy in xenograft models of small-cell lung cancer and lymphoma and potentiates the activity of numerous cytotoxic agents. Here we describe the biological activity of A-385358, a small molecule with relative selectivity for binding to Bcl-X(L) versus Bcl-2 (K(i)s of 0.80 and 67 nmol/L for Bcl-X(L) and Bcl-2, respectively). This compound efficiently enters cells and co-localizes with the mitochondrial membrane. Although A-385358 shows relatively modest single-agent cytotoxic activity against most tumor cell lines, it has an EC(50) of <500 nmol/L in cells dependent on Bcl-X(L) for survival. In addition, A-385358 enhances the in vitro cytotoxic activity of numerous chemotherapeutic agents (paclitaxel, etoposide, cisplatin, and doxorubicin) in several tumor cell lines. In A549 non-small-cell lung cancer cells, A-385358 potentiates the activity of paclitaxel by as much as 25-fold. Importantly, A-385358 also potentiated the activity of paclitaxel in vivo. Significant inhibition of tumor growth was observed when A-385358 was added to maximally tolerated or half maximally tolerated doses of paclitaxel in the A549 xenograft model. In tumors, the combination therapy also resulted in a significant increase in mitotic arrest followed by apoptosis relative to paclitaxel monotherapy.

Activity of the Bcl-2 Family Inhibitor ABT-263 in a Panel of Small Cell Lung Cancer Xenograft Models

Purpose: The purpose of this study was to characterize the activity of the Bcl-2 protein family inhibitor ABT-263 in a panel of small cell lung cancer (SCLC) xenograft models. Experimental Design: A panel of 11 SCLC xenograft models was established to evaluate the efficacy of ABT-263. Single agent activity was examined on a continuous dosing schedule in each of these models. The H146 model was used to further evaluate dose and schedule, comparison to standard cytotoxic agents, and induction of apoptosis. Results: ABT-263 exhibited a range of antitumor activity, leading to complete tumor regression in several models. Significant regressions of tumors as large as 1 cc were also observed. The efficacy of ABT-263 was also quite durable; in several cases, minimal tumor regrowth was noted several weeks after the cessation of treatment. Antitumor effects were equal or superior to that of several clinically approved cytotoxic agents. Regression of large established tumors was observed through several cycles of therapy and efficacy was retained in a Pgp-1 overexpressing line. Significant efficacy was observed on several dose and therapeutic schedules and was associated with significant induction of apoptosis. Conclusions: ABT-263 is a potent, orally bioavailable inhibitor of Bcl-2 family proteins that has recently entered clinical trials. The efficacy data reported here suggest that SCLC is a promising area of clinical investigation with this agent.

ABT-888 Confers Broad In vivo Activity in Combination with Temozolomide in Diverse Tumors

Purpose: ABT-888, currently in phase 2 trials, is a potent oral poly(ADP-ribose) polymerase inhibitor that enhances the activity of multiple DNA-damaging agents, including temozolomide (TMZ). We investigated ABT-888+TMZ combination therapy in multiple xenograft models representing various human tumors having different responses to TMZ. Experimental Design: ABT-888+TMZ efficacy in xenograft tumors implanted in subcutaneous, orthotopic, and metastatic sites was assessed by tumor burden, expression of poly(ADP-ribose) polymer, and O6-methylguanine methyltransferase (MGMT). Results: Varying levels of ABT-888+TMZ sensitivity were evident across a broad histologic spectrum of models (55-100% tumor growth inhibition) in B-cell lymphoma, small cell lung carcinoma, non–small cell lung carcinoma, pancreatic, ovarian, breast, and prostate xenografts, including numerous regressions. Combination efficacy in otherwise TMZ nonresponsive tumors suggests that TMZ resistance may be overcome by poly(ADP-ribose) polymerase inhibition. Profound ABT-888+TMZ efficacy was seen in experimental metastases models that acquired resistance to TMZ. Moreover, TMZ resistance was overcome in crossover treatments, indicating that combination therapy may overcome acquired TMZ resistance. Neither tumor MGMT, mismatch repair, nor poly(ADP-ribose) polymer correlated with the degree of sensitivity to ABT-888+TMZ. Conclusions: Robust ABT-888+TMZ efficacy is observed across a spectrum of tumor types, including orthotopic and metastatic implantation. As many TMZ nonresponsive tumors proved sensitive to ABT-888+TMZ, this novel combination may broaden the clinical use of TMZ beyond melanoma and glioma. Although TMZ resistance may be influenced by MGMT, neither MGMT nor other mechanisms of TMZ resistance (mismatch repair) precluded sensitivity to ABT-888+TMZ. Underlying mechanisms of TMZ resistance in these models are not completely understood but likely involve mechanisms independent of MGMT.(Clin Cancer Res 2009;15(23):7277–90)

The Bcl-2 inhibitor ABT-263 enhances the response of multiple chemotherapeutic regimens in hematologic tumors in vivo

PurposeThis study was designed to test the ability of the Bcl-2 family inhibitor ABT-263 to potentiate commonly used chemotherapeutic agents and regimens in hematologic tumor models.MethodsModels of B-cell lymphoma and multiple myeloma were tested in vitro and in vivo with ABT-263 in combination with standard chemotherapeutic regimens, including VAP, CHOP and R-CHOP, as well as single cytotoxic agents including etoposide, rituximab, bortezomib and cyclophosphamide. Alterations in Bcl-2 family member expression patterns were analyzed to define mechanisms of potentiation.ResultsABT-263 was additive with etoposide, vincristine and VAP in vitro in the diffuse large B-cell lymphoma line (DLBCL) DoHH-2, while rituximab potentiated its activity in SuDHL-4. Bortezomib strongly synergized with ABT-263 in the mantle cell lymphoma line Granta 519. Treatment of DoHH-2 with etoposide was associated with an increase in Puma expression, while bortezomib upregulated Noxa expression in Granta 519. Combination of ABT-263 with cytotoxic agents demonstrated superior tumor growth inhibition and delay in multiple models versus cytotoxic therapy alone, along with significant improvements in tumor response rates.ConclusionsInhibition of the Bcl-2 family of proteins by ABT-263 enhances the cytotoxicity of multiple chemotherapeutics in hematologic tumors and represents a promising addition to the therapeutic arsenal for treatment of these diseases.

Synthesis and Biological Evaluation of 2-Indolyloxazolines as a New Class of Tubulin Polymerization Inhibitors. Discovery of A-289099 as an Orally Active Antitumor Agent

A series of indole containing oxazolines has been discovered as a result of structural modifications of the lead compound A-105972. The compounds exert their anticancer activity through inhibition of tubulin polymerization by binding at the colchicine site. A-289099 was identified as an orally active antimitotic agent active against various cancer cell lines including those that express the MDR phenotype. The anticancer activity, pharmacokinetics, and an efficient and enantioselective synthesis of A-289099 are described.

Synthesis and evaluation of a new generation of orally efficacious benzimidazole-based poly(ADP-ribose) polymerase-1 (PARP-1) inhibitors as anticancer agents.

Small molecule inhibitors of PARP-1 have been pursued by various organizations as potential therapeutic agents either capable of sensitizing cytotoxic treatments or acting as stand-alone agents to combat cancer. As one of the strategies to expand our portfolio of PARP-1 inhibitors, we pursued unsaturated heterocycles to replace the saturated cyclic amine derivatives appended to the benzimidazole core. Not only did a variety of these new generation compounds maintain high enzymatic potency, many of them also displayed robust cellular activity. For example, the enzymatic IC(50) and cellular EC(50) values were as low as 1 nM or below. Compounds 24 (EC(50) = 3.7 nM) and 44 (EC(50) = 7.8 nM), featuring an oxadiazole and a pyridine moiety, respectively, demonstrated balanced potency and PK profiles. In addition, these two molecules exhibited potent oral in vivo efficacy in potentiating the cytotoxic agent temozolomide in a B16F10 murine melanoma model.

ABT-263 and rapamycin act cooperatively to kill lymphoma cells in vitro and in vivo

ABT-263 is a potent, orally bioavailable inhibitor of the antiapoptotic Bcl-2 family members Bcl-2, Bcl-xL, and Bcl-w, which is currently in phase I clinical trials. Previous work has shown that this compound has low nanomolar cell-killing activity in a variety of lymphoma and leukemia cell lines, many of which overexpress Bcl-2 through a variety of mechanisms. Rapamycin is a macrolide antibiotic that inhibits the mammalian target of rapamycin complex, leading to cell cycle arrest and inhibition of protein translation. Rapamycin (and its analogues) has shown activity in a variety of tumor cell lines primarily through induction of cell cycle arrest. Activity has also been shown clinically in mantle cell lymphoma and advanced renal cell carcinoma. Here, we show that treatment of the follicular lymphoma lines DoHH-2 and SuDHL-4 with 100 nmol/L rapamycin induces substantial G0-G1 arrest. Addition of as little as 39 nmol/L ABT-263 to the rapamycin regimen induced a 3-fold increase in sub-G0 cells. Combination of these agents also led to a significant increase in Annexin V staining over ABT-263 alone. In xenograft models of these tumors, rapamycin induced a largely cytostatic response in the DoHH-2 and SuDHL-4 models. Coadministration with ABT-263 induced significant tumor regression, with DoHH-2 and SuDHL-4 tumors showing 100% overall response rates. Apoptosis in these tumors was significantly enhanced by combination therapy as measured by staining with an antibody specific for cleaved caspase-3. These data suggest that combination of ABT-263 and rapamycin or its analogues represents a promising therapeutic strategy for the treatment of lymphoma. [Mol Cancer Ther 2008;7(10):3265–74]