Parsaclisib Is a Next-Generation Phosphoinositide 3-Kinase δ Inhibitor with Reduced Hepatotoxicity and Potent Antitumor and Immunomodulatory Activities in Models of B-Cell Malignancy.

The clinical use of first-generation phosphoinositide 3-kinase (PI3K)δ inhibitors in B-cell malignancies is hampered by hepatotoxicity, requiring dose reduction, treatment interruption, and/or discontinuation of therapy. In addition, potential molecular mechanisms by which resistance to this class of drugs occurs have not been investigated. Parsaclisib (INCB050465) is a potent and selective next-generation PI3Kδ inhibitor that differs in structure from first-generation PI3Kδ inhibitors and has shown encouraging anti-B-cell tumor activity and reduced hepatotoxicity in phase 1/2 clinical studies. Here, we present preclinical data demonstrating parsaclisib as a potent inhibitor of PI3Kδ with over 1000-fold selectivity against other class 1 PI3K isozymes. Parsaclisib directly blocks PI3K signaling-mediated cell proliferation in B-cell lines in vitro and in vivo and indirectly controls tumor growth by lessening immunosuppression through regulatory T-cell inhibition in a syngeneic lymphoma model. Diffuse large B-cell lymphoma cell lines overexpressing MYC were insensitive to proliferation blockade via PI3Kδ signaling inhibition by parsaclisib, but their proliferative activities were reduced by suppression of MYC gene transcription. Molecular structure analysis of the first- and next-generation PI3Kδ inhibitors combined with clinical observation suggests that hepatotoxicity seen with the first-generation inhibitors could result from a structure-related off-target effect. Parsaclisib is currently being evaluated in multiple phase 2 clinical trials as a therapy against various hematologic malignancies of B-cell origin (NCT03126019, NCT02998476, NCT03235544, NCT03144674, and NCT02018861). SIGNIFICANCE STATEMENT: The preclinical properties described here provide the mechanism of action and support clinical investigations of parsaclisib as a therapy for B-cell malignancies. MYC overexpression was identified as a resistance mechanism to parsaclisib in DLBCL cells, which may be useful in guiding further translational studies for the selection of patients with DLBCL who might benefit from PI3Kδ inhibitor treatment in future trials. Hepatotoxicity associated with first-generation PI3Kδ inhibitors may be an off-target effect of that class of compounds.

The Novel Bromodomain and Extraterminal Domain Inhibitor INCB054329 Induces Vulnerabilities in Myeloma Cells That Inform Rational Combination Strategies.

PURPOSE: Bromodomain and extraterminal domain (BET) proteins regulate the expression of many cancer-associated genes and pathways; BET inhibitors have demonstrated activity in diverse models of hematologic and solid tumors. We report the preclinical characterization of INCB054329, a structurally distinct BET inhibitor that has been investigated in phase I clinical trials. EXPERIMENTAL DESIGN: We used multiple myeloma models to investigate vulnerabilities created by INCB054329 treatment that could inform rational combinations. RESULTS: In addition to c-MYC, INCB054329 decreased expression of oncogenes FGFR3 and NSD2/MMSET/WHSC1, which are deregulated in t(4;14)-rearranged cell lines. The profound suppression of FGFR3 sensitized the t(4;14)-positive cell line OPM-2 to combined treatment with a fibroblast growth factor receptor inhibitor in vivo. In addition, we show that BET inhibition across multiple myeloma cell lines resulted in suppressed interleukin (IL)-6 Janus kinase-signal transducers and activators of transcription (JAK-STAT) signaling. INCB054329 displaced binding of BRD4 to the promoter of IL6 receptor (IL6R) leading to reduced levels of IL6R and diminished signaling through STAT3. Combination with JAK inhibitors (ruxolitinib or itacitinib) further reduced JAK-STAT signaling and synergized to inhibit myeloma cell growth in vitro and in vivo. This combination potentiated tumor growth inhibition in vivo, even in the MM1.S model of myeloma that is not intrinsically sensitive to JAK inhibition alone. CONCLUSIONS: Preclinical data reveal insights into vulnerabilities created in myeloma cells by BET protein inhibition and potential strategies that can be leveraged in clinical studies to enhance the activity of INCB054329.

Validation of standards for quantitative assessment of JAK2 c.1849G>T (p.V617F) allele burden analysis in clinical samples.

The substitution of valine with phenylalanine at amino acid 617 of the Janus kinase 2 (JAK2) gene (JAK2 p.V617F) occurs in a high proportion of patients with myeloproliferative neoplasms (MPNs). The ability to accurately measure JAK2 p.V617F allele burden is of great interest given the diagnostic relevance of the mutation and the ongoing clinical evaluation of JAK inhibitors. A main hurdle in developing quantitative assays for allele burden measurement is the unavailability of accurate standards for both assay validation and use in a standard curve for quantification. We describe our approach to the validation of standards for quantitative assessment of JAK2 p.V617F allele burden in clinical MPN samples. These standards were used in two JAK2 p.V617F assays, which were used to support clinical studies of ruxolitinib (Jakafi(®)) in myelofibrosis, a real-time polymerase chain reaction assay for initial screening of all samples, and a novel single-nucleotide polymorphism typing (SNaPshot)-based assay for samples with less than 5% mutant allele burden. Comparisons of allele burden data from clinical samples generated with these assays show a high degree of concordance with each other and with a pyrosequencing-based assay used for clinical reporting from an independent laboratory, thus providing independent validation to the accuracy of these standards.

Selective inhibition of IDO1 effectively regulates mediators of antitumor immunity.

Indoleamine 2,3-dioxygenase-1 (IDO1; IDO) mediates oxidative cleavage of tryptophan, an amino acid essential for cell proliferation and survival. IDO1 inhibition is proposed to have therapeutic potential in immunodeficiency-associated abnormalities, including cancer. Here, we describe INCB024360, a novel IDO1 inhibitor, and investigate its roles in regulating various immune cells and therapeutic potential as an anticancer agent. In cellular assays, INCB024360 selectively inhibits human IDO1 with IC(50) values of approximately 10nM, demonstrating little activity against other related enzymes such as IDO2 or tryptophan 2,3-dioxygenase (TDO). In coculture systems of human allogeneic lymphocytes with dendritic cells (DCs) or tumor cells, INCB024360 inhibition of IDO1 promotes T and natural killer (NK)-cell growth, increases IFN-gamma production, and reduces conversion to regulatory T (T(reg))-like cells. IDO1 induction triggers DC apoptosis, whereas INCB024360 reverses this and increases the number of CD86(high) DCs, potentially representing a novel mechanism by which IDO1 inhibition activates T cells. Furthermore, IDO1 regulation differs in DCs versus tumor cells. Consistent with its effects in vitro, administration of INCB024360 to tumor-bearing mice significantly inhibits tumor growth in a lymphocyte-dependent manner. Analysis of plasma kynurenine/tryptophan levels in patients with cancer affirms that the IDO pathway is activated in multiple tumor types. Collectively, the data suggest that selective inhibition of IDO1 may represent an attractive cancer therapeutic strategy via up-regulation of cellular immunity.

Hydroxyamidine inhibitors of indoleamine-2,3-dioxygenase potently suppress systemic tryptophan catabolism and the growth of IDO-expressing tumors.

Malignant tumors arise, in part, because the immune system does not adequately recognize and destroy them. Expression of indoleamine-2,3-dioxygenase (IDO; IDO1), a rate-limiting enzyme in the catabolism of tryptophan into kynurenine, contributes to this immune evasion. Here we describe the effects of systemic IDO inhibition using orally active hydroxyamidine small molecule inhibitors. A single dose of INCB023843 or INCB024360 results in efficient and durable suppression of Ido1 activity in the plasma of treated mice and dogs, the former to levels seen in Ido1-deficient mice. Hydroxyamidines potently suppress tryptophan metabolism in vitro in CT26 colon carcinoma and PAN02 pancreatic carcinoma cells and in vivo in tumors and their draining lymph nodes. Repeated administration of these IDO1 inhibitors impedes tumor growth in a dose- and lymphocyte-dependent fashion and is well tolerated in efficacy and preclinical toxicology studies. Substantiating the fundamental role of tumor cell-derived IDO expression, hydroxyamidines control the growth of IDO-expressing tumors in Ido1-deficient mice. These activities can be attributed, at least partially, to the increased immunoreactivity of lymphocytes found in tumors and their draining lymph nodes and to the reduction in tumor-associated regulatory T cells. INCB024360, a potent IDO1 inhibitor with desirable pharmaceutical properties, is poised to start clinical trials in cancer patients.

Preclinical characterization of the selective JAK1/2 inhibitor INCB018424: therapeutic implications for the treatment of myeloproliferative neoplasms.

Constitutive JAK2 activation in hematopoietic cells by the JAK2V617F mutation recapitulates myeloproliferative neoplasm (MPN) phenotypes in mice, establishing JAK2 inhibition as a potential therapeutic strategy. Although most polycythemia vera patients carry the JAK2V617F mutation, half of those with essential thrombocythemia or primary myelofibrosis do not, suggesting alternative mechanisms for constitutive JAK-STAT signaling in MPNs. Most patients with primary myelofibrosis have elevated levels of JAK-dependent proinflammatory cytokines (eg, interleukin-6) consistent with our observation of JAK1 hyperactivation. Accordingly, we evaluated the effectiveness of selective JAK1/2 inhibition in experimental models relevant to MPNs and report on the effects of INCB018424, the first potent, selective, oral JAK1/JAK2 inhibitor to enter the clinic. INCB018424 inhibited interleukin-6 signaling (50% inhibitory concentration [IC(50)] = 281nM), and proliferation of JAK2V617F(+) Ba/F3 cells (IC(50) = 127nM). In primary cultures, INCB018424 preferentially suppressed erythroid progenitor colony formation from JAK2V617F(+) polycythemia vera patients (IC(50) = 67nM) versus healthy donors (IC(50) > 400nM). In a mouse model of JAK2V617F(+) MPN, oral INCB018424 markedly reduced splenomegaly and circulating levels of inflammatory cytokines, and preferentially eliminated neoplastic cells, resulting in significantly prolonged survival without myelosuppressive or immunosuppressive effects. Preliminary clinical results support these preclinical data and establish INCB018424 as a promising oral agent for the treatment of MPNs.

Combined inhibition of Janus kinase 1/2 for the treatment of JAK2V617F-driven neoplasms: selective effects on mutant cells and improvements in measures of disease severity.

PURPOSE: Deregulation of the Janus kinase-signal transducers and activators of transcription (JAK-STAT) pathway is a hallmark for the Philadelphia chromosome-negative myeloproliferative diseases polycythemia vera, essential thrombocythemia, and primary myelofibrosis. We tested the efficacy of a selective JAK1/2 inhibitor in cellular and in vivo models of JAK2-driven malignancy. EXPERIMENTAL DESIGN: A novel inhibitor of JAK1/2 was characterized using kinase assays. Cellular effects of this compound were measured in cell lines bearing the JAK2V617F or JAK1V658F mutation, and its antiproliferative activity against primary polycythemiavera patient cells was determined using clonogenic assays. Antineoplastic activity in vivo was determined using a JAK2V617F-driven xenograft model, and effects of the compound on survival, organomegaly, body weight, and disease-associated inflammatory markers were measured. RESULTS: INCB16562 potently inhibited proliferation of cell lines and primary cells from PV patients carrying the JAK2V617F or JAK1V658F mutation by blocking JAK-STAT signaling and inducing apoptosis. In vivo, INCB16562 reduced malignant cell burden, reversed splenomegaly and normalized splenic architecture, improved body weight gains, and extended survival in a model of JAK2V617F-driven hematologic malignancy. Moreover, these mice suffered from markedly elevated levels of inflammatory cytokines, similar to advanced myeloproliferative disease patients, which was reversed upon treatment. CONCLUSIONS: These data showed that administration of the dual JAK1/2 inhibitor INCB16562 reduces malignant cell burden, normalizes spleen size and architecture, suppresses inflammatory cytokines, improves weight gain, and extends survival in a rodent model of JAK2V617F-driven hematologic malignancy. Thus, selective inhibitors of JAK1 and JAK2 represent a novel therapy for the patients with myeloproliferative diseases and other neoplasms associated with JAK dysregulation.

Gene content of the 750-kb critical region for mouse embryonic ectoderm lethal tcl-w5.

Mice homozygous for the t(w5) allele arrest at gastrulation from defects associated with embryonic ectoderm development. The mutated gene has been genetically closely linked to the H-2K locus in the mouse MHC region, flanked by markers H-2Pb and D17Mit147. Aiming at the positional cloning of the mutated gene, we constructed a BAC contig spanning about 1 Mb of the genomic region. On the basis of our mapping and sequencing analysis of the BACs combined with public genome data, EST database searches, and gene prediction programs, we delimit the 1.06 cM of the t(w5) critical region to 750 kb, and infer 36 genes (1/20 kb) encoded in the interval. All of the 33 genes tested were confirmed as expressed in embryonic tissues by RT-PCR analyses, and in many cases by EST expression profiles as well. Thus, this highly gene-rich region is essentially totally transcribed during early development and provides priority candidates to be screened for the t(w5) embryonic lesion.