Pirtobrutinib after a Covalent BTK Inhibitor in Chronic Lymphocytic Leukemia.

BACKGROUND: Patients with chronic lymphocytic leukemia (CLL) or small lymphocytic lymphoma (SLL) have poor outcomes after the failure of covalent Bruton's tyrosine kinase (BTK) inhibitor treatment, and new therapeutic options are needed. Pirtobrutinib, a highly selective, noncovalent (reversible) BTK inhibitor, was designed to reestablish BTK inhibition. METHODS: We conducted a phase 1-2 trial in which patients with relapsed or refractory B-cell cancers received pirtobrutinib. Here, we report efficacy results among patients with CLL or SLL who had previously received a BTK inhibitor as well as safety results among all the patients with CLL or SLL. The primary end point was an overall response (partial response or better) as assessed by independent review. Secondary end points included progression-free survival and safety. RESULTS: A total of 317 patients with CLL or SLL received pirtobrutinib, including 247 who had previously received a BTK inhibitor. Among these 247 patients, the median number of previous lines of therapy was 3 (range, 1 to 11), and 100 patients (40.5%) had also received a B-cell lymphoma 2 (BCL2) inhibitor such as venetoclax. The percentage of patients with an overall response to pirtobrutinib was 73.3% (95% confidence interval [CI], 67.3 to 78.7), and the percentage was 82.2% (95% CI, 76.8 to 86.7) when partial response with lymphocytosis was included. The median progression-free survival was 19.6 months (95% CI, 16.9 to 22.1). Among all 317 patients with CLL or SLL who received pirtobrutinib, the most common adverse events were infections (in 71.0%), bleeding (in 42.6%), and neutropenia (in 32.5%). At a median duration of treatment of 16.5 months (range, 0.2 to 39.9), some adverse events that are typically associated with BTK inhibitors occurred relatively infrequently, including hypertension (in 14.2% of patients), atrial fibrillation or flutter (in 3.8%), and major hemorrhage (in 2.2%). Only 9 of 317 patients (2.8%) discontinued pirtobrutinib owing to a treatment-related adverse event. CONCLUSIONS: In this trial, pirtobrutinib showed efficacy in patients with heavily pretreated CLL or SLL who had received a covalent BTK inhibitor. The most common adverse events were infections, bleeding, and neutropenia. (Funded by Loxo Oncology; BRUIN ClinicalTrials.gov number, NCT03740529.).

Pirtobrutinib in Covalent Bruton Tyrosine Kinase Inhibitor Pretreated Mantle-Cell Lymphoma.

PURPOSE: Pirtobrutinib is a highly selective, noncovalent (reversible) Bruton tyrosine kinase inhibitor (BTKi). We report the safety and efficacy of pirtobrutinib in patients with covalent Bruton tyrosine kinase inhibitor (cBTKi) pretreated mantle-cell lymphoma (MCL), a population with poor prognosis. METHODS: Patients with cBTKi pretreated relapsed/refractory (R/R) MCL received pirtobrutinib monotherapy in a multicenter phase I/II trial (BRUIN; ClinicalTrials.gov identifier: NCT03740529). Efficacy was assessed in the first 90 consecutively enrolled patients who met criteria for inclusion in the primary efficacy cohort. The primary end point was overall response rate (ORR). Secondary end points included duration of response (DOR) and safety. RESULTS: The median patient age was 70 years (range, 46-87), the median prior lines of therapy was 3 (range, 1-8), 82.2% had discontinued a prior cBTKi because of disease progression, and 77.8% had intermediate- or high-risk simplified MCL International Prognostic Index score. The ORR was 57.8% (95% CI, 46.9 to 68.1), including 20.0% complete responses (n = 18). At a median follow-up of 12 months, the median DOR was 21.6 months (95% CI, 7.5 to not reached). The 6- and 12-month estimated DOR rates were 73.6% and 57.1%, respectively. In the MCL safety cohort (n = 164), the most common treatment-emergent adverse events (TEAEs) were fatigue (29.9%), diarrhea (21.3%), and dyspnea (16.5%). Grade ≥3 TEAEs of hemorrhage (3.7%) and atrial fibrillation/flutter (1.2%) were less common. Only 3% of patients discontinued pirtobrutinib because of a treatment-related adverse event. CONCLUSION: Pirtobrutinib is a first-in-class novel noncovalent (reversible) BTKi and the first BTKi of any kind to demonstrate durable efficacy after prior cBTKi therapy in heavily pretreated R/R MCL. Pirtobrutinib was well tolerated with low rates of treatment discontinuation because of toxicity.

MAGI-2 scaffold protein is critical for kidney barrier function.

MAGUK Inverted 2 (MAGI-2) is a PTEN-interacting scaffold protein implicated in cancer on the basis of rare, recurrent genomic translocations and deletions in various tumors. In the renal glomerulus, MAGI-2 is exclusively expressed in podocytes, specialized cells forming part of the glomerular filter, where it interacts with the slit diaphragm protein nephrin. To further explore MAGI-2 function, we generated Magi-2-KO mice through homologous recombination by targeting an exon common to all three alternative splice variants. Magi-2 null mice presented with progressive proteinuria as early as 2 wk postnatally, which coincided with loss of nephrin expression in the glomeruli. Magi-2-null kidneys revealed diffuse podocyte foot process effacement and focal podocyte hypertrophy by 3 wk of age, as well as progressive podocyte loss. By 5.5 wk, coinciding with a near-complete loss of podocytes, Magi-2-null mice developed diffuse glomerular extracapillary epithelial cell proliferations, and died of renal failure by 3 mo of age. As confirmed by immunohistochemical analysis, the proliferative cell populations in glomerular lesions were exclusively composed of activated parietal epithelial cells (PECs). Our results reveal that MAGI-2 is required for the integrity of the kidney filter and podocyte survival. Moreover, we demonstrate that PECs can be activated to form glomerular lesions resembling a noninflammatory glomerulopathy with extensive extracapillary proliferation, sometimes resembling crescents, following rapid and severe podocyte loss.

Glucocorticoid receptor confers resistance to antiandrogens by bypassing androgen receptor blockade.

The treatment of advanced prostate cancer has been transformed by novel antiandrogen therapies such as enzalutamide. Here, we identify induction of glucocorticoid receptor (GR) expression as a common feature of drug-resistant tumors in a credentialed preclinical model, a finding also confirmed in patient samples. GR substituted for the androgen receptor (AR) to activate a similar but distinguishable set of target genes and was necessary for maintenance of the resistant phenotype. The GR agonist dexamethasone was sufficient to confer enzalutamide resistance, whereas a GR antagonist restored sensitivity. Acute AR inhibition resulted in GR upregulation in a subset of prostate cancer cells due to relief of AR-mediated feedback repression of GR expression. These findings establish a mechanism of escape from AR blockade through expansion of cells primed to drive AR target genes via an alternative nuclear receptor upon drug exposure.

Overcoming mutation-based resistance to antiandrogens with rational drug design.

The second-generation antiandrogen enzalutamide was recently approved for patients with castration-resistant prostate cancer. Despite its success, the duration of response is often limited. For previous antiandrogens, one mechanism of resistance is mutation of the androgen receptor (AR). To prospectively identify AR mutations that might confer resistance to enzalutamide, we performed a reporter-based mutagenesis screen and identified a novel mutation, F876L, which converted enzalutamide into an AR agonist. Ectopic expression of AR F876L rescued the growth inhibition of enzalutamide treatment. Molecular dynamics simulations performed on antiandrogen-AR complexes suggested a mechanism by which the F876L substitution alleviates antagonism through repositioning of the coactivator recruiting helix 12. This model then provided the rationale for a focused chemical screen which, based on existing antiandrogen scaffolds, identified three novel compounds that effectively antagonized AR F876L (and AR WT) to suppress the growth of prostate cancer cells resistant to enzalutamide. DOI:http://dx.doi.org/10.7554/eLife.00499.001.

Constitutively active androgen receptor splice variants expressed in castration-resistant prostate cancer require full-length androgen receptor.

Androgen receptor (AR) splice variants lacking the ligand binding domain (ARVs), originally isolated from prostate cancer cell lines derived from a single patient, are detected in normal and malignant human prostate tissue, with the highest levels observed in late stage, castration-resistant prostate cancer. The most studied variant (called AR-V7 or AR3) activates AR reporter genes in the absence of ligand and therefore, could play a role in castration resistance. To explore the range of potential ARVs, we screened additional human and murine prostate cancer models using conventional and next generation sequencing technologies and detected several structurally diverse AR isoforms. Some, like AR-V7/AR3, display gain of function, whereas others have dominant interfering activity. We also find that ARV expression increases acutely in response to androgen withdrawal, is suppressed by testosterone, and in some models, is coupled to full-length AR (AR-FL) mRNA production. As expected, constitutively active, ligand-independent ARVs such as AR-V7/AR3 are sufficient to confer anchorage-independent (in vitro) and castration-resistant (in vivo) growth. Surprisingly, this growth is blocked by ligand binding domain-targeted antiandrogens, such as MDV3100, or by selective siRNA silencing of AR-FL, indicating that the growth-promoting effects of ARVs are mediated through AR-FL. These data indicate that the increase in ARV expression in castrate-resistant prostate cancer is an acute response to castration rather than clonal expansion of castration or antiandrogen-resistant cells expressing gain of function ARVs, and furthermore, they provide a strategy to overcome ARV function in the clinic.

Transient potent BCR-ABL inhibition is sufficient to commit chronic myeloid leukemia cells irreversibly to apoptosis.

The BCR-ABL inhibitor dasatinib achieves clinical remissions in chronic myeloid leukemia (CML) patients using a dosing schedule that achieves potent but transient BCR-ABL inhibition. In vitro, transient potent BCR-ABL inhibition with either dasatinib or imatinib is cytotoxic to CML cell lines, as is transient potent EGFR inhibition with erlotinib in a lung cancer cell line. Cytotoxicity correlates with the magnitude as well as the duration of kinase inhibition. Moreover, cytotoxicity with transient potent target inhibition is equivalent to prolonged target inhibition and in both cases is associated with BIM activation and rescued by BCL-2 overexpression. In CML patients receiving dasatinib once daily, response correlates with the magnitude of BCR-ABL kinase inhibition, thereby demonstrating the potential clinical utility of intermittent potent kinase inhibitor therapy.