For Better or Worse: The Potential for Dose Limiting the On-Target Toxicity of PI 3-Kinase Inhibitors

Inhibitors of phosphoinositide 3-kinase (PI3K) and phosphoinositide 3-kinase-related protein kinase family (PIKK)

Phosphoinositide 3-kinases (PI3K) and phosphoinositide 3-kinase-related protein kinases (PIKK) are two structurally related families of kinases that play vital roles in cell growth and DNA damage repair. Dysfunction of PIKK members and aberrant stimulation of the PI3K/AKT/mTOR signalling pathway are linked to a plethora of diseases including cancer. In recent decades, numerous inhibitors related to the PI3K/AKT/mTOR signalling have made great strides in cancer treatment, like copanlisib and sirolimus. Notably, most of the PIKK inhibitors (such as VX-970 and M3814) related to DNA damage response have also shown good efficacy in clinical trials. However, these drugs still require a suitable combination therapy to overcome drug resistance or improve antitumor activity. Based on the aforementioned facts, we summarised the efficacy of PIKK, PI3K, and AKT inhibitors in the therapy of human malignancies and the resistance mechanisms of targeted therapy, in order to provide deeper insights into cancer treatment.

Targeting phosphatidylinositol-3-kinase for inhibiting maxillary bone resorption

Previous studies have suggested a role of phosphatidylinositol-3-kinase gamma (PI3Kγ) in bone remodeling, but the mechanism remains undefined. Here, we explored the contribution of PI3Kγ in the resorption of maxillary bone and dental roots using models of orthodontic tooth movement (OTM), orthodontic-induced inflammatory root resorption, and rapid maxillary expansion (RME). PI3Kγ-deficient mice (PI3Kγ-/- ), mice with loss of PI3Kγ kinase activity (PI3KγKD/KD ) and C57BL/6 mice treated with a PI3Kγ inhibitor (AS605240) and respective controls were used. The maxillary bones of PI3Kγ-/- , PI3KγKD/KD , and C57BL/6 mice treated with AS605240 showed an improvement of bone quality compared to their controls, resulting in reduction of the OTM and RME in all experimental groups. PI3Kγ-/- mice exhibited increased root volume and decreased odontoclasts counts. Consistently, the pharmacological blockade or genetic deletion of PI3K resulted in increased numbers of osteoblasts and reduction in osteoclasts during OTM. There was an augmented expression of Runt-related transcription factor 2 (Runx2) and alkaline phosphatase (Alp), a reduction of interleukin-6 (Il-6), as well as a lack of responsiveness of receptor activator of nuclear factor kappa-Β (Rank) in PI3Kγ-/- and PI3KγKD/KD mice compared to control mice. The maxillary bones of PI3Kγ-/- animals showed reduced p-Akt expression. In vitro, bone marrow cells treated with AS605240 and cells from PI3Kγ-/- mice exhibited significant augment of osteoblast mineralization and less osteoclast differentiation. The PI3Kγ/Akt axis is pivotal for bone remodeling by providing negative and positive signals for the differentiation of osteoclasts and osteoblasts, respectively.

Dominant Role of PI3K p110α over p110β in Insulin and β-Adrenergic Receptor Signalling

Attribution of specific roles to the two ubiquitously expressed PI 3-kinase (PI3K) isoforms p110α and p110β in biological functions they have been implicated, such as in insulin signalling, has been challenging. While p110α has been demonstrated to be the principal isoform activated downstream of the insulin receptor, several studies have provided evidence for a role of p110β. Here we have used isoform-selective inhibitors to estimate the relative contribution of each of these isoforms in insulin signalling in adipocytes, which are a cell type with essential roles in regulation of metabolism at the systemic level. Consistent with previous genetic and pharmacological studies, we found that p110α is the principal isoform activated downstream of the insulin receptor under physiological conditions. p110α interaction with Ras enhanced the strength of p110α activation by insulin. However, this interaction did not account for the selectivity for p110α over p110β in insulin signalling. We also demonstrate that p110α is the principal isoform activated downstream of the β-adrenergic receptor (β-AR), another important signalling pathway in metabolic regulation, through a mechanism involving activation of the cAMP effector molecule EPAC1. This study offers further insights in the role of PI3K isoforms in the regulation of energy metabolism with implications for the therapeutic application of selective inhibitors of these isoforms.

Inhibition of histone acetyltransferase function radiosensitizes CREBBP/EP300 mutants via repression of homologous recombination, potentially targeting a gain of function

Despite radiation forming the curative backbone of over 50% of malignancies, there are no genomically-driven radiosensitizers for clinical use. Herein we perform in vivo shRNA screening to identify targets generally associated with radiation response as well as those exhibiting a genomic dependency. This identifies the histone acetyltransferases CREBBP/EP300 as a target for radiosensitization in combination with radiation in cognate mutant tumors. Further in vitro and in vivo studies confirm this phenomenon to be due to repression of homologous recombination following DNA damage and reproducible using chemical inhibition of histone acetyltransferase (HAT), but not bromodomain function. Selected mutations in CREBBP lead to a hyperacetylated state that increases CBP and BRCA1 acetylation, representing a gain of function targeted by HAT inhibition. Additionally, mutations in CREBBP/EP300 are associated with recurrence following radiation in squamous cell carcinoma cohorts. These findings provide both a mechanism of resistance and the potential for genomically-driven treatment.

Integrated safety analysis of umbralisib, a dual PI3Kδ/CK1ε inhibitor, in relapsed/refractory lymphoid malignancies

Umbralisib is a unique PI3Kδ/casein kinase-1ε inhibitor with a tolerable safety profile in relapsed/refractory lymphoid malignancies.

Low rates of immune-mediated toxicities were observed with umbralisib.

Phosphoinositide 3-kinase-δ (PI3Kδ) inhibitors are active in lymphoid malignancies, although associated toxicities can limit their use. Umbralisib is a dual inhibitor of PI3Kδ and casein kinase-1ε (CK1ε). This study analyzed integrated comprehensive toxicity data from 4 open-label, phase 1 and 2 studies that included 371 adult patients (median age, 67 years) with relapsed/refractory non-Hodgkin lymphoma (follicular lymphoma [n = 147]; marginal zone lymphoma [n = 82]; diffuse large B-cell lymphoma/mantle cell lymphoma [n = 74]; chronic lymphocytic leukemia [n = 43]; and other tumor types [n = 25]) who were treated with the recommended phase 2 dose of umbralisib 800 mg or higher once daily. At data cutoff, median duration of umbralisib treatment was 5.9 months (range, 0.1-75.1 months), and 107 patients (28.8%) received umbralisib for ≥12 months. Any-grade treatment-emergent adverse events (AEs) occurred in 366 (98.7%) of 371 patients, with the most frequent being diarrhea (52.3%), nausea (41.5%), and fatigue (31.8%). Grade 3 or higher treatment-emergent AEs occurred in 189 (50.9%) of 371 patients and included neutropenia (11.3%), diarrhea (7.3%), and increased aminotransferase levels (5.7%). Treatment-emergent serious AEs occurred in 95 (25.6%) of 371 patients. AEs of special interest were limited and included pneumonia (29 of 371 [7.8%]), noninfectious colitis (9 of 371 [2.4%]), and pneumonitis (4 of 371 [1.1%]). AEs led to discontinuation of umbralisib in 51 patients (13.7%). Four patients (1.1%) died of AEs, none of which was deemed related to umbralisib. No cumulative toxicities were reported. The favorable long-term tolerability profile and low rates of immune-mediated toxicities support the potential use of umbralisib for the benefit of a broad population of patients with lymphoid malignancies.

PI3Kα inhibitor CYH33 triggers antitumor immunity in murine breast cancer by activating CD8+T cells and promoting fatty acid metabolism

BACKGROUND

The phosphatidylinositol 3-kinase (PI3K) is frequently hyperactivated in cancer and plays important roles in both malignant and immune cells. The effect of PI3Kα inhibitors on the tumor microenvironment (TME) remains largely unknown. Here, we investigated the modulation of the TME by a clinical PI3Kα-specific inhibitor CYH33.

METHODS

The activity of CYH33 against a panel of murine tumors in the immune-competent context or athymic mice was detected. Single-cell RNA sequencing and multi-parameter flow cytometry were performed to determine the immune profiling of TME. The effect of CYH33 on immune cells was conducted with primary murine cells.

RESULTS

CYH33 exhibited more potent antitumor activity in immune-competent context. CYH33 enhanced the infiltration and activation of CD8+T and CD4+T cells, while attenuating M2-like macrophages and regulatory CD4+T cells. Increase in memory T cells was confirmed by the induction of long-term immune memory on CYH33 treatment. Mechanistically, CYH33 relieved the suppressed expansion of CD8+T cells via preferential polarization of the macrophages to the M1 phenotype. CYH33 promoted fatty acid (FA) metabolism in the TME, while FA enhanced the activity of CD8+T cells in vitro. The combination of CYH33 with the FA synthase (FASN) inhibitor C75 synergistically inhibited tumor growth with enhanced host immunity.

CONCLUSIONS

CYH33 induces immune activation and synergizes with FASN inhibitor to further promote the antitumor immunity, which gains novel insights into how PI3K inhibitors exert their activity by modulating TME and provides a rationale for the concurrent targeting of PI3K and FASN in breast cancer treatment.

Alpelisib-Induced Diabetic Ketoacidosis: A Case Report and Review of Literature

Objective

To report the first case of diabetic ketoacidosis (DKA) and its management in a patient with diet-controlled prediabetes and metastatic breast cancer treated with alpelisib, a PI3K (phosphatidylinosiotol-3-kinase) inhibitor.

Methods

Literature on the topic is reviewed. The case is that of a 66-year-old female with diet-controlled prediabetes and metastatic breast carcinoma who had initiated alpelisib 2 weeks prior to being admitted for diabetic ketoacidosis.

Results

Admission laboratory examination revealed a blood sugar of 1137 mg/dL, an anion gap of 25, large ketones in urine, and positive acetone in serum. The HbA1c level was 9.4% (79 mmol/mol) on admission, which had been 6.3% (45 mmol/mol) seven months earlier. She was discharged on subcutaneous insulin and instructed to discontinue alpelisib. Alpelisib was restarted 2 days later, which exacerbated her hyperglycemia within 24 hours. In the following months, her hyperglycemia was successfully managed with insulin and a SGLT 2 inhibitor. Unfortunately, her breast cancer progressed, ultimately leading to discontinuation of alpelisib. Blood sugar levels returned to a nondiabetic range upon discontinuation of alpelisib, and she is currently off all antihyperglycemic agents.

Conclusion

Although PI3KCA inhibitors remain a promising drug in patients with metastatic breast cancer who have not responded to previous treatment, patients must be closely monitored for adverse effects such as hyperglycemia. Hyperglycemia could be a potentially limiting side effect of alpelisib. The optimal management of hyperglycemia induced by alpelisib warrants further research.

Phosphatidylinositol 3-Kinase δ-Specific Inhibitor-Induced Changes in the Ovary and Testis in the Sprague Dawley Rat and Cynomolgus Monkey

Phosphatidylinositol 3-kinase (PI3K) δ is a lipid kinase primarily found in leukocytes, which regulates important cell functions. AMG2519493 was a PI3K δ-specific inhibitor in development for treatment of various inflammatory diseases. AMG2519493-related changes in the male and/or female reproductive organs were observed in the 1- and 3-month oral repeat dose toxicology studies in the rat and cynomolgus monkey. Hemorrhagic corpora lutea cysts and increased incidence of corpora lutea cysts without hemorrhage were observed in the ovaries at supra pharmacological doses in the rat. A decrease in seminiferous germ cells in the testis, indicative of spermatogenesis maturation arrest, was observed in both the rat and cynomolgus monkey. Although the characteristics were comparable, the drug systemic exposures associated with the testicular changes were very different between the 2 species. In the rat, the testicular change was only observed at supra pharmacologic exposure. Isotype assessment of PI3K signaling in rat spermatogonia in vitro indicated a role for PI3K β, but not δ, in the c Kit/PI3K/protein kinase B signaling pathway. Therefore, changes in both the ovary and testis of the rat were considered due to off target effect as they only occurred at suprapharmacologic exposure. In contrast, the testicular changes in the cynomolgus monkey (decrease in seminiferous germ cells) occurred at very low doses associated with PI3K δ-specific inhibition, indicating that the PI3K δ isoform may be important in spermatogenesis maturation in the cynomolgus monkey. Our results suggest species-related differences in PI3K isoform-specific control on reproductive organs.

Properties of FDA-approved small molecule phosphatidylinositol 3-kinase inhibitors prescribed for the treatment of malignancies

The discovery of the phosphatidylinositol 3-kinase (PI 3-kinase) pathway was a major advance in understanding eukaryotic signal transduction. The high frequency of PI 3-kinase pathway mutations in many cancers stimulated the development of drugs targeting these oncogenic mutants. The PI 3-kinases are divided into three classes and Class I PI 3-kinases, which catalyze the phosphorylation of phosphatidylinositol-4,5-bisphosphate (PI-4,5-P2) to generate phosphatidylinositol-3,4,5-trisphosphate (PIP3), are the main subject of this review. The class I PI 3-kinases are made up of p110α, p110β, p110δ, and p110γ catalytic subunits. These catalytic subunits are constitutively bound to regulatory subunits (p85α, p85β, p55γ, p101, and p87 proteins). The p85/p55 regulatory subunits heterodimerize with p110α or p110δ thereby forming complexes that are regulated chiefly by receptor protein-tyrosine kinases. The p101 and p87 subunits heterodimerize with p110γ to form complexes that are regulated mainly by G protein-coupled receptors (GPCRs). Complexes containing the p110β subunit are activated by receptor protein-tyrosine kinases as well as GPCRs. Following the generation of PIP3, the AKT and mTOR protein-serine/threonine kinases are activated leading to cell growth, proliferation, and survival. Like protein kinases, the PI 3-kinase domains consist of a bilobed structure connected by a hinge-linker segment. ATP and most PI 3-kinase and protein kinase inhibitors form hydrogen bonds with hinge residues. The small and large lobes of PI 3-kinases and protein kinases have a very similar three-dimensional structure called the protein kinase fold. Both PI 3-kinases and eukaryotic protein kinases possess an activation segment that begins with a DFG triad (Asp-Phe-Gly); the activation segment of protein kinases usually ends with an APE (Ala-Pro-Glu) signature while that of PI 3-kinases ends with a PFxLT (Pro-Phe-Xxx-Leu-Thr) signature. Dormant PI 3-kinases have a collapsed activation loop and active PI 3-kinases have an extended activation loop. The distance between the α-carbon atom of the DFG-D residue at the beginning of the activation loop and that of the PFxLT-F residue at the end of the activation loop in dormant PI 3-kinases is about 13 Å; this distance in active PI 3-kinases is about 18 Å. The protein kinase catalytic loop has an HRD (His-Arg-Asp) signature while that of the PI 3-kinases reverses the order with a DRH triad. Alpelisib is an orally effective FDA-approved PI 3-kinase-α inhibitor used for the treatment of breast cancer. Copanlisib, duvelisib, idelalisib, and umbralisib are PI 3-kinase-δ inhibitors that are approved for the third-line treatment of follicular lymphomas and other hematological disorders. Copanlisib is also a potent inhibitor of PI 3-kinase-α. Of the five approved drugs, all are orally bioavailable except copanlisib. Idelalisib interacts with the active conformation of PI 3-kinase-δ and is classified as a type I inhibitor. Alpelisib and copanlisib interact with inactive PI 3-kinase-α and PI 3-kinase-γ, respectively, and are classified as a type I½ antagonists. Except for umbralisib with a molecular weight of 571.5, all five drugs conform to the Lipinski rule of five for oral effectiveness. Copanlisib, however, must be given intravenously. Alpelisib and copanlisib inhibit PI 3-kinase-α, which is involved in insulin signaling, and both drugs promote insulin-resistance and produce hyperglycemia. The five FDA-approved PI 3-kinase inhibitors produce significant on-target toxicities, more so than many approved protein kinase antagonists. The development of PI 3-kinase inhibitors with fewer toxicities is an important long-term therapeutic goal.

Umbralisib, a Dual PI3Kδ/CK1ε Inhibitor in Patients With Relapsed or Refractory Indolent Lymphoma

PURPOSE

Phosphatidylinositol-3-kinase (PI3K) inhibitors have shown activity in relapsed or refractory (R/R) indolent non-Hodgkin lymphoma (iNHL). PI3K inhibitors have been hampered by poor long-term tolerability and toxicity, which interfere with continuous use. Umbralisib, a dual inhibitor of PI3Kδ/casein kinase-1ε, exhibits improved selectivity for PI3Kδ compared with other PI3K inhibitors. This phase IIb trial was designed to evaluate the efficacy and safety of umbralisib in patients with R/R iNHL.

PATIENTS AND METHODS

In this multicohort, open-label, phase IIb study, 208 patients with R/R marginal zone, follicular, or small lymphocytic lymphoma (MZL, FL, or SLL) unresponsive to prior treatments (≥ 1 MZL; ≥ 2 FL/SLL), including ≥ 1 anti-CD20-based therapy, were administered umbralisib 800 mg orally once daily until disease progression, unacceptable toxicity, or study withdrawal. Primary end point is overall response rate; secondary end points include time to response, duration of response, progression-free survival, and safety.

RESULTS

The median follow-up is 27.7 months (efficacy) and 21.4 months (safety). The overall response rate was 47.1%, and tumor reduction occurred in 86.4% of patients. The median time to response was 2.7-4.6 months. The median duration of response was not reached for MZL, 11.1 months for FL, and 18.3 months for SLL. Median progression-free survival was not reached for MZL, 10.6 months for FL, and 20.9 months for SLL. At least one grade ≥ 3 treatment-emergent adverse event (TEAE) was reported in 53.4% of patients. TEAEs led to umbralisib discontinuation in 32 patients (15.4%). A total of 31 patients (14.9%) discontinued because of a treatment-related adverse event. Grade ≥ 3 TEAEs reported in ≥ 10% of patients: neutropenia (11.5%) and diarrhea (10.1%). Increased ALT/AST (grade ≥ 3) occurred in 6.7%/7.2% of patients.

CONCLUSION

Umbralisib achieved meaningful clinical activity in heavily pretreated patients with iNHL. The safety profile was manageable, with a relatively low incidence of immune-mediated toxicities and adverse event-related discontinuations.

Discovery of a Potent and Selective PI3Kδ Inhibitor (S)-2,4-Diamino-6-((1-(7-fluoro-1-(4-fluorophenyl)-4-oxo-3-phenyl-4H-quinolizin-2-yl)ethyl)amino)pyrimidine-5-carbonitrile with Improved Pharmacokinetic Profile and Superior Efficacy in Hematological Cancer Models

PI3Kδ inhibitors have been approved for B-cell malignancies like CLL, small lymphocytic lymphoma, and so forth. However, currently available PI3Kδ inhibitors are nonoptimal, showing weakness against at least one of the several important properties: potency, isoform selectivity, and/or pharmacokinetic profile. To come up with a PI3Kδ inhibitor that overcomes all these deficiencies, a pharmacophoric expansion strategy was employed. Herein, we describe a systematic transformation of a "three-blade propeller" shaped lead, 2,3-disubstituted quinolizinone 11, through a 1,2-disubstituted quinolizinone 20 to a novel "four-blade propeller" shaped 1,2,3-trisubstituted quinolizinone 34. Compound 34 has excellent potency, isoform selectivity, metabolic stability across species, and exhibited a favorable pharmacokinetic profile. Compound 34 also demonstrated a differentiated efficacy profile in human germinal center B and activated B cell-DLBCL cell lines and xenograft models. Compound 34 qualifies for further evaluation as a candidate for monotherapy or in combination with other targeted agents in DLBCLs and other forms of iNHL.

Cotargeting BET proteins overcomes resistance arising from PI3K/mTOR blockade-induced protumorigenic senescence in colorectal cancer

Therapeutics targeting the phosphatidylinositol 3-kinase/mammalian target of rapamycin (PI3K/mTOR) pathway initially produce potent antitumor effects, but resistance frequently occurs. Using a phosphoproteome analysis, we found that colorectal cancer (CRC) cells exhibit resistance against PI3K/mTOR inhibition through feedback activation of multiple receptor tyrosine kinases, and their downstream focal adhesion kinase, Src and extracellular signal-regulated kinases signaling. Unexpectedly, PI3K/mTOR blockade causes senescence, mediated by the activation of the stress kinase p38. The senescent cancer cells induce the secretion of various cytokines and this senescence-associated secretome increases migration and invasion capabilities of CRC cells. We found that cotargeting PI3K/mTOR and bromodomain and extra-terminal domain can suppress activation of many oncogenic kinases involved in resistance to the PI3K/mTOR inhibition, induce cell death in vitro and tumor regression in vivo, and further prolong the survival of xenograft models. Our findings provide a rationale for a novel therapeutic strategy to overcome resistance to the PI3K/mTOR inhibitors in CRC.

Evolution of Molecular Targets in Melanoma Treatment

Melanoma is the deadliest type of skin cancers, accounting for more than 80% of skin cancer mortality. Although melanoma was known very early in the history of medicine, treatment for this disease had remained largely the same until very recently. Previous treatment options, including removal surgery and systemic chemotherapy, offered little benefit in extending the survival of melanoma patients. However, the last decade has seen breakthroughs in melanoma treatment, which all emerged following new insight into the oncogenic signaling of melanoma. This paper reviewed the evolution of drug targets for melanoma treatment based on the emergence of novel findings in the molecular signaling of melanoma. One of the findings that are most influential in melanoma treatment is that more than 50% of melanoma tumors contain BRAF mutations. This is fundamental for the development of BRAF inhibitors, which is the first group of drugs that significantly improves the overall survival of melanoma patients compared to the traditional chemotherapeutic dacarbazine. More recently, findings of the role of immune checkpoint molecules such as CTLA-4 and PD1/PD-L1 in melanoma biology have led to the development of a new therapeutic category: immune checkpoint inhibitors, which, for the first time in the history of cancer treatment, produced a durable response in a subset of melanoma patients. However, as this paper discussed next, there is still an unmet need for melanoma treatment. A significant population of patients did not respond to either BRAF inhibitors or immune checkpoint inhibitors. Of those patients who gained an initial response from those therapies, a remarkable percentage would develop drug resistance even when MEK inhibitors were added to the treatment. Finally, this paper discusses some possible targets for melanoma treatment.

Evolving treatment strategies of brain metastases from breast cancer: current status and future direction

Remarkable progress in breast cancer treatment has improved patient survival, resulting in an increased incidence of brain metastasis (BM). Current treatment options for BM are limited and are generally used for palliative purposes. Historically, local treatment, consisting of radiotherapy and surgery, is the standard of care due to delivery limitations of systemic treatments through the blood-brain barrier. However, as novel biological mechanisms for tumors and BM have been discovered, several innovative systemic agents, such as small-molecular-targeted therapy and immunotherapy, have begun to change the treatment paradigm. In addition, efforts to maximize antitumor effects have been attempted using combination therapy, informed by tumor biology. In this comprehensive review, we will highlight various clinical trials investigating the treatment of BM in breast cancer patients, discuss presently available treatment options, and suggest potential directions of future therapeutic targets.