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.

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.