Gemtuzumab Ozogamicin Treatment Results in Decreased Proliferation and Differentiation of Human Megakaryocytes but Does Not Inhibit Mature Platelet Function.

Mylotarg (Gemtuzumab ozogamicin [GO]), an antibody drug conjugate comprising a CD33-directed antibody linked to calicheamicin, is approved for use in certain acute myeloid leukemia patients. Following reports of prolonged thrombocytopenia and hemorrhagic events in a subset of patients, a detailed series of in vitro and ex vivo studies was performed at the request of regulators, both to look at the effects of GO on platelet production and to determine whether treatment with GO was likely to affect platelet aggregation under a variety of conditions. Treatment with GO resulted in cellular cytotoxicity and/or decreased differentiation during human megakaryocyte development. However, GO did not impair platelet aggregation under the experimental conditions evaluated. Ultimately, the effect of GO on megakaryocyte development observed in our studies was determined to have no impact on the risk-benefit assessment in the intended patient population, as thrombocytopenia is a known side effect of GO, and monitoring of platelet counts in patients is already strongly recommended.

Comparative Analyses of Poly(ADP-Ribose) Polymerase Inhibitors.

Poly(ADP-ribose) polymerase inhibitors (PARPi) are approved as monotherapies in BRCA1/2-mutated (mBRCA1/2) metastatic breast and ovarian cancers, and in advanced pancreatic and metastatic castration-resistant prostate cancers. Differential safety profiles across PARPi necessitate improved mechanistic understanding of inhibitor differences, especially with expansion of PARPi indications and drug combinations. Here, we report in vitro evaluations of PARPi (-/+ PARP trapper temozolomide, TMZ) with reference to total clinical mean concentration average or maximum (tCavg, tCmax), to elucidate contributions of primary pharmacology and structural differences to clinical efficacy and safety. In biochemical assays, rucaparib and niraparib demonstrated off-target secondary pharmacology activities, and in selectivity assays, talazoparib, olaparib, and rucaparib inhibited a broader panel of PARP enzymes. In donor-derived human bone marrow mononuclear cells, only olaparib both increased early apoptosis and decreased the cell viability half inhibitory concentration (IC50) at ≤ tCavg, whereas other PARPi only did so in the presence of TMZ. In cancer cell lines with DNA damage repair mutations, all PARPi decreased cell viability in H1048 but not TK6 cells, and only talazoparib decreased cell growth in DU145 cells at ≤ tCavg concentrations. When combined with low dose TMZ, only talazoparib left-shifted the functional consequences of PARP trapping (S-phase arrest, apoptosis, S-phase double-stranded breaks) and reduced cell viability/growth in TK6 and DU145 cell lines at ≤ tCavg, whereas the other inhibitors required high-dose TMZ. Our study suggests structural differences across PARPi may contribute to differences in PARP selectivity and off-target activities, which along with distinct pharmacokinetic properties, may influence inhibitor-specific toxicities in patients.

Liver Toxicity Observed With Lorlatinib When Combined With Strong CYP3A Inducers: Evaluation of Cynomolgus Monkey as a Nonclinical Model for Assessing the Mechanism of Combinational Toxicity.

Lorlatinib is a potent small-molecule anaplastic lymphoma kinase inhibitor approved for the treatment of patients with nonsmall cell lung cancer. In a drug-drug interaction study in healthy human participants, liver enzyme elevations were observed when a single 100 mg dose of lorlatinib was administered after multiple doses of rifampin, a strong cytochrome P450 (CYP) 3A inducer and a pregnane X receptor (PXR) agonist. A series of in vitro and in vivo studies were conducted to evaluate potential mechanisms for the observed clinical toxicity. To investigate the involvement of CYP3A and/or PXR in the observed liver toxicity, studies were conducted in cynomolgus monkeys administered lorlatinib alone or with coadministration of multiple doses of known CYP3A inducers that are predominantly PXR agonists (rifampin, St. John's wort) or predominantly constitutive androstane receptor agonists (carbamazepine, phenytoin) and a net CYP3A inhibitory PXR agonist (ritonavir). Results from the investigative studies identified cynomolgus monkeys as a pharmacologically relevant nonclinical model, which recapitulated the elevated liver function test results observed in humans. Furthermore, liver toxicity was only observed in this model when lorlatinib was coadministered with strong CYP3A inducers, and the effects were not restricted to, or exclusively dependent upon, a PXR activation mechanism. These results generated mechanistic insights on the liver enzyme elevations observed in the clinical drug-drug interaction study and provided guidance on appropriate product safety label for lorlatinib.

PF-06804103, A Site-specific Anti-HER2 Antibody-Drug Conjugate for the Treatment of HER2-expressing Breast, Gastric, and Lung Cancers.

The approval of ado-trastuzumab emtansine (T-DM1) in HER2+ metastatic breast cancer validated HER2 as a target for HER2-specific antibody-drug conjugates (ADC). Despite its demonstrated clinical efficacy, certain inherent properties within T-DM1 hamper this compound from achieving the full potential of targeting HER2-expressing solid tumors with ADCs. Here, we detail the discovery of PF-06804103, an anti-HER2 ADC designed to have a widened therapeutic window compared with T-DM1. We utilized an empirical conjugation site screening campaign to identify the engineered ĸkK183C and K290C residues as those that maximized in vivo ADC stability, efficacy, and safety for a four drug-antibody ratio (DAR) ADC with this linker-payload combination. PF-06804103 incorporates the following novel design elements: (i) a new auristatin payload with optimized pharmacodynamic properties, (ii) a cleavable linker for optimized payload release and enhanced antitumor efficacy, and (iii) an engineered cysteine site-specific conjugation approach that overcomes the traditional safety liabilities of conventional conjugates and generates a homogenous drug product with a DAR of 4. PF-06804103 shows (i) an enhanced efficacy against low HER2-expressing breast, gastric, and lung tumor models, (ii) overcomes in vitro- and in vivo-acquired T-DM1 resistance, and (iii) an improved safety profile by enhancing ADC stability, pharmacokinetic parameters, and reducing off-target toxicities. Herein, we showcase our platform approach in optimizing ADC design, resulting in the generation of the anti-HER2 ADC, PF-06804103. The design elements of identifying novel sites of conjugation employed in this study serve as a platform for developing optimized ADCs against other tumor-specific targets.

Enzalutamide and Apalutamide: In Vitro Chemical Reactivity Studies and Activity in a Mouse Drug Allergy Model.

Enzalutamide and apalutamide are two androgen receptor inhibitors approved for the treatment of castration-resistant prostate cancer (CRPC) and nonmetastatic castration-resistant prostate cancer (nmCRPC), respectively. Apalutamide is associated with an increased incidence of skin rash above the placebo groups in the SPARTAN trial in nmCRPC and in the TITAN trial in metastatic castration-sensitive prostate cancer patients. On the contrary, the rate of skin rash across all clinical trials (including PROSPER [nmCRPC]) for enzalutamide is similar to the placebo. We hypothesized that the apalutamide-associated increased skin rash in patients could be linked to a structural difference. The 2-cyanophenyl and dimethyl moieties in enzalutamide are substituted in apalutamide with 2-cyanopyridine and cyclobutyl, respectively. In our evaluations, the 2-cyanopyridine moiety of apalutamide was chemically reactive with the thiol nucleophile glutathione, resulting in rearranged thiazoline products. Radiolabeled apalutamide, but not radiolabeled enzalutamide, was shown to react with mouse and human plasma proteins. Thiol nucleophiles decreased the extent of covalent binding to the model protein bovine serum albumin, whereas amine and alcohol nucleophiles had no effect, suggesting reactivity with cysteine of proteins. Subcutaneous administration of apalutamide dose dependently increased lymphocyte cellularity in draining lymph nodes in a mouse drug allergy model (MDAM). Enzalutamide, and its known analogue RD162 in which the cyanophenyl was retained but the dimethyl was replaced by cyclobutyl, demonstrated substantially less covalent binding activity and negative results in the MDAM assay. Collectively, these data support the hypothesis that the 2-cyanopyridine moiety in apalutamide may react with cysteine in proteins forming haptens, which may trigger an immune response, as indicated by the activity of apalutamide in the MDAM assay, which in turn may be leading to increased potential for skin rash versus placebo in patients in the SPARTAN and TITAN clinical trials.

Myocarditis in Cynomolgus Monkeys Following Treatment with Immune Checkpoint Inhibitors.

PURPOSE: Immune checkpoint inhibitors (ICI) targeting PD1, PDL1, or CTLA4 are associated with immune-related adverse events (irAE) in multiple organ systems including myocarditis. The pathogenesis and early diagnostic markers for ICI-induced myocarditis are poorly understood, and there is currently a lack of laboratory animal model to enhance our understanding. We aimed to develop such a model using cynomolgus monkeys. EXPERIMENTAL DESIGN: Chinese-origin cynomolgus monkeys were dosed intravenously with vehicle or nivolumab 20 mg/kg plus ipilimumab 15 mg/kg once weekly and euthanized on day 29. RESULTS: Multiple organ toxicities were observed in cynomolgus monkeys, and were characterized by loose feces, lymphadenopathy, and mononuclear cell infiltrations of varying severity in heart, colon, kidneys, liver, salivary glands, and endocrine organs. Increased proliferation of CD4+ and CD8+ T lymphocytes as well as an increase in activated T cells and central memory T cells in the blood, spleen, and lymph nodes, were observed. Transcriptomic analysis suggested increased migration and activation of T cells and increased phagocytosis and antigen presentation in the heart. Mononuclear cell infiltration in myocardium was comprised primarily of T cells, with lower numbers of macrophages and occasional B cells, and was associated with minimal cardiomyocyte degeneration as well as increases in cardiac troponin-I and NT-pro-BNP. Morphologically, cardiac lesions in our monkey model are similar to the reported ICI myocarditis in humans. CONCLUSIONS: We have developed a monkey model characterized by multiple organ toxicities including myocarditis. This model may provide insight into the immune mechanisms and facilitate biomarker identification for ICI-associated irAEs.

Intestinal Toxicity in Rats Following Administration of CDK4/6 Inhibitors Is Independent of Primary Pharmacology.

Recently three different cyclin-dependent kinase 4 and 6 (CDK4/6) dual inhibitors were approved for the treatment of breast cancer (palbociclib, ribociclib, and abemaciclib), all of which offer comparable therapeutic benefits. Their safety profiles, however, are different. For example, neutropenia is observed at varying incidences in patients treated with these drugs; however, it is the most common adverse event for palbociclib and ribociclib, whereas diarrhea is the most common adverse event observed in patients treated with abemaciclib. To understand the mechanism of diarrhea observed with these drugs and in an effort to guide the development of safer drugs, we compared the effects of oral administration of palbociclib, ribociclib, and abemaciclib on the gastrointestinal tract of rats using doses intended to produce comparable CDK4/6 inhibition. Rats administered abemaciclib, but not palbociclib or ribociclib, had fecal alterations, unique histopathologic findings, and distinctive changes in intestinal gene expression. Morphologic changes in the intestine were characterized by proliferation of crypt cells, loss of goblet cells, poorly differentiated and degenerating enterocytes with loss of microvilli, and mucosal inflammation. In the jejunum of abemaciclib-treated rats, downregulation of enterocyte membrane transporters and upregulation of genes associated with cell proliferation were observed, consistent with activation of the Wnt pathway and downstream transcriptional regulation. Among these CDK4/6 inhibitors, intestinal toxicity was unique to rats treated with abemaciclib, suggesting a mechanism of toxicity not due to primary pharmacology (CDK4/6 inhibition), but to activity at secondary pharmacologic targets.

CDK4/6 Inhibition on Glucose and Pancreatic Beta Cell Homeostasis in Young and Aged Rats.

Genetic deletion of cyclin-dependent kinase 4 (Cdk4) is associated with pancreatic beta cell loss and glucose dysregulation in rodents. Palbociclib, one of the first selective CDK4/6 inhibitors approved for the treatment of advanced breast cancer, is currently being investigated as an adjuvant treatment in patients with early-stage breast cancer and in a variety of cancers covering a wide-range of patient populations. Hence, longer chronic toxicity studies were necessary to further examine its safety profile. The effects of different doses and duration of palbociclib administration on glucose and beta cell homeostasis in young (two months) versus aged (12 months) rats was compared. Glucose dysregulation, due to pancreatic beta cell degeneration, was observed in young rats administered the highest dose of palbociclib for 6 months. Abnormal pancreatic islet histology and activation of the endoplasmic reticulum stress response in beta cells were detected after shorter administration with high-dose palbociclib in young rats. To test the hypothesis that palbociclib-associated inhibition of beta cell proliferation will more profoundly affect younger animals that have not achieved replicative quiescence, we administered high-dose palbociclib to aged rats for 6 months. In contrast to the young rats, despite equivalent exposures to palbociclib, no evidence of impaired glucose tolerance, hypoinsulinemia, beta cell vacuolization, or beta cell loss was seen in aged rats. Palbociclib administration induces beta cell failure in young but not aged rats.Implications: Although adult humans receiving palbociclib have not displayed detectable adverse effects on glucose metabolism, the risk of beta cell failure in children remains unexplored. Mol Cancer Res; 15(11); 1531-41. ©2017 AACR.

Liver Microvascular Injury and Thrombocytopenia of Antibody-Calicheamicin Conjugates in Cynomolgus Monkeys-Mechanism and Monitoring.

Purpose: Adverse reactions reported in patients treated with antibody-calicheamicin conjugates such as gemtuzumab ozogamicin (Mylotarg) and inotuzumab ozogamicin include thrombocytopenia and sinusoidal obstruction syndrome (SOS). The objective of this experimental work was to investigate the mechanism for thrombocytopenia, characterize the liver injury, and identify potential safety biomarkers.Experimental Design: Cynomolgus monkeys were dosed intravenously at 6 mg/m2/dose once every 3 weeks with a nonbinding antibody-calicheamicin conjugate (PF-0259) containing the same linker-payload as gemtuzumab ozogamicin and inotuzumab ozogamicin. Monkeys were necropsied 48 hours after the first administration (day 3) or 3 weeks after the third administration (day 63).Results: PF-0259 induced acute thrombocytopenia (up to 86% platelet reduction) with nadirs on days 3 to 4. There was no indication of effects on megakaryocytes in bone marrow or activation of platelets in peripheral blood. Microscopic evaluation of liver from animals necropsied on day 3 demonstrated midzonal degeneration and loss of sinusoidal endothelial cells (SECs) associated with marked platelet accumulation in sinusoids. Liver histopathology on day 63 showed variable endothelial recovery and progression to a combination of sinusoidal capillarization and sinusoidal dilation/hepatocellular atrophy, consistent with early SOS. Among biomarkers evaluated, there were early and sustained increases in serum hyaluronic acid (HA) that correlated well with serum aspartate aminotransferase and liver microscopic changes, suggesting that HA may be a sensitive diagnostic marker of the liver microvascular injury.Conclusions: These data support the conclusion that target-independent damage to liver SECs may be responsible for acute thrombocytopenia (through platelet sequestration in liver sinusoids) and development of SOS. Clin Cancer Res; 23(7); 1760-70. ©2016 AACR.

Mechanistic Investigation of Bone Marrow Suppression Associated with Palbociclib and its Differentiation from Cytotoxic Chemotherapies.

PURPOSE: Palbociclib (PD-0332991) is the first selective cyclin-dependent kinase (CDK) 4/6 inhibitor approved for metastatic breast cancer. Hematologic effects, especially neutropenia, are dose-limiting adverse events for palbociclib in humans. EXPERIMENTAL DESIGN: Reversible hematologic effects and bone marrow hypocellularity have been identified in toxicology studies in rats and dogs after palbociclib treatment. To understand the mechanism by which the hematologic toxicity occurs, and to further differentiate it from the myelotoxicity caused by cytotoxic chemotherapeutic agents, anin vitroassay using human bone marrow mononuclear cells (hBMNC) was utilized. RESULTS: This work demonstrated that palbociclib-induced bone marrow suppression occurred through cell-cycle arrest, with no apoptosis at clinically relevant concentrations, was not lineage-specific, and was reversible upon palbociclib withdrawal. In contrast, treatment with chemotherapeutic agents (paclitaxel and doxorubicin) resulted in DNA damage and apoptotic cell death in hBMNCs. In the presence or absence of the antiestrogen, palbociclib-treated hBMNCs did not become senescent and resumed proliferation following palbociclib withdrawal, consistent with pharmacologic quiescence. The breast cancer cells, MCF-7, conversely, became senescent following palbociclib or antiestrogen treatment with additive effects in combination and remained arrested in the presence of antiestrogen. CONCLUSIONS: Palbociclib causes reversible bone marrow suppression, clearly differentiating it from apoptotic cell death caused by cytotoxic chemotherapeutic agents. This study also distinguished the cell-cycle arresting action of palbociclib on normal bone marrow cells from the senescent effects observed in breast cancer cells. These results shed light on the mechanism and support risk management of palbociclib-induced bone marrow toxicity in the clinic.

Pharmacological effects of nicotine on norepinephrine metabolism in rat brown adipose tissue: relevance to nicotinic therapies for smoking cessation.

In a two-year carcinogenicity study with administration of high doses of the partial nicotinic agonist varenicline (recently approved for smoking cessation), mediastinal hibernomas occurred in three male rats. To investigate potential mechanisms for partial and full nicotinic agonists to contribute to development of hibernomas, the effects of nicotine on rat brown adipose tissue (BAT) were studied. Male and female rats were administered nicotine at doses of 0, 0.3, and 1 mg/kg subcutaneously for fourteen days. Intrathoracic (mediastinal periaortic and mediastinal perithymic) BAT and interscapular BAT were examined microscopically, and determinations of uncoupling protein-1 (UCP-1) expression and norepinephrine (NE) content were made. Additionally, NE turnover was measured in mediastinal periaortic and perithymic BAT. Nicotine (1 mg/kg) administration resulted in decreased vacuolation only in mediastinal periaortic and mediastinal perithymic BAT of males and elevated UCP-1 in mediastinal periaortic BAT of males and females. Increased NE content occurred only in mediastinal periaortic BAT of males given 0.3 and 1 mg/kg doses, whereas NE turnover was decreased in both males and females given 1 mg/kg. Together, these data demonstrate that nicotine primarily affects mediastinal BAT in male rats, consistent with the gender and location of the hibernomas observed in the two-year carcinogenicity study.

Pharmacological characterization of CP-547,632, a novel vascular endothelial growth factor receptor-2 tyrosine kinase inhibitor for cancer therapy.

Signaling through vascular endothelial growth factor (VEGF) receptors (VEGFRs) is a key pathway initiating endothelial cell proliferation and migration resulting in angiogenesis, a requirement for human tumor growth and metastasis. Abrogation of signaling through VEGFR by a variety of approaches has been demonstrated to inhibit angiogenesis and tumor growth. Small molecule inhibitors of VEGFR tyrosine kinase have been shown to inhibit angiogenesis, inhibit tumor growth, and prevent metastases. Our goal was to discover and characterize an p.o. active VEGFR-2 small molecule inhibitor. A novel isothiazole, CP-547,632, was identified as a potent inhibitor of the VEGFR-2 and basic fibroblast growth factor (FGF) kinases (IC(50) = 11 and 9 nM, respectively). It is selective relative to epidermal growth factor receptor, platelet-derived growth factor beta, and other related TKs. It also inhibits VEGF-stimulated autophosphorylation of VEGFR-2 in a whole cell assay with an IC(50) value of 6 nM. After oral administration of CP-547,632 to mice bearing NIH3T3/H-ras tumors, VEGFR-2 phosphorylation in tumors was inhibited in a dose-dependent fashion (EC(50) = 590 ng/ml). These plasma concentrations correlated well with the observed concentrations of the compound necessary to inhibit VEGF-induced corneal angiogenesis in BALB/c mice. A sponge angiogenesis assay was used to directly compare the inhibitory activities of CP-547,632 against FGF receptor 2 or VEGFR-2; this compound potently inhibits both basic FGF and VEGF-induced angiogenesis in vivo. The antitumor efficacy of this agent was evaluated after once daily p.o. administration to athymic mice bearing human xenografts and resulted in as much as 85% tumor growth inhibition. CP-547,632 is a well-tolerated, orally-bioavailable inhibitor presently under clinical investigation for the treatment of human malignancies.