The phosphatidylinositol 3-kinase (PI3K) pathway is a key cell signaling node whose dysregulation commonly results in the transformation of normal cells into cancer cells. The role of PI3K and its impact in cancer is well documented and, as a result, it has and continues to be the focus of attention for anticancer therapeutic drug discovery. The PI3Ks mostly implicated in human cancers include PI3K-α, PI3K-β, PI3K-δ, and PI3K-γ. Current experimental therapeutic approaches aim at either inhibiting all PI3K isozymes simultaneously (pan inhibition) or attempting to inhibit a specific isozyme (isoform selective).
SignalRx is developing novel anticancer therapeutic agents targeting PI3K implementing these 2 strategies (pan- and isoform-selective PI3K inhibition) while simultaneously inhibiting additional cancer-relevant biological targets. The choice of the “other” target is driven by an evolving understanding of cancer biology. Our approach with these enhanced anti-cancer agents will provide improved therapeutic efficacy and deter the development of resistance thus increasing time to disease progression or increasing survival.
The SignalRx R & D pipeline is shown below followed by a brief description of each program:
SF1126, a Phase II ready anticancer therapeutic, is the only vascular-targeted multi-kinase inhibitor for advanced solid tumors and it is available for partnering to continue development in Phase II trials. Phase I trial results demonstrate that SF1126 has an excellent therapeutic window with excellent tolerability and safety. These results have been published in Eur. J. Cancer 2012, Vol. 48, Issue 18, pages 3319-3327 (doi: 10.1016/j.ejca.2012.06.027) and summarized in the tables below:
Further clinical trials are in preparation including a phase II adult cancer patient trial and a phase Ib pediatric cancer patient trial.
The planned phase II adult cancer patient trial is a “bucket” trial whereby it is limited to patients whose genomic makeup of their cancer suggests they would benefit from inhibition of the PI3K/mTOR pathway. This approach is driven by the evolving understanding of cancer biology whereby the more we learn the more complex it becomes to effectively treat cancers because of alternate mechanism of resistance. Thus, a more efficient regulatory path would limit the treatment regimen to those cancers whose biology would predict more likely responses because the cancers are more dependent on certain cell signaling pathways. In our case with SF1126 those most likely to respond would include cancer patients with activating mutations in the alpha isoform of PI3K (PIK3CA gene) or patients with loss of PTEN function. In addition to measuring time to progression and survival times the design of the SF1126 phase II trial allows us to perform extensive molecular profiling of tumor tissue to identify a biomarker signature that will predict responsiveness or resistance to this therapy
SignalRx’s has a strong commitment to treating pediatric cancer even though it is a small market because of the enormous unmet medical needs. Our commitment comes from the fact that pediatric cancers are largely last in line for new technologies in part because of the small market. Pediatric cancers are first in line for SignalRx’s SF1126 technology and will be informed by our previous experience with SF1126 in adult cancer patients to jumpstart the approval process in pediatric cancers. The planned SF1126 phase 1b is a dose escalation pediatric cancer trial. This phase 1b trial will determine the toxicity of SF1126 in a pediatric cohort and is designed to also evaluate a Health Related Quality of Life (HRQoL) endpoint in patients treated with SF1126. The design of this phase 1b pediatric trial of SF1126 also allows us to perform extensive molecular profiling of tumor tissue to identify a biomarker signature that will predict responsiveness or resistance to this therapy.
SF2523 is a new potent pan-PI3K small molecule inhibitor that belongs to a new proprietary compound class: 5-morpholino-7H-thieno[3,2-b]pyran-7-one (TP). This candidate exhibits a remarkable kinase selectivity inhibiting potently only 4 out of the 232 non-PI3K kinases at the enzyme level, showing broad potency against 19 cancer cell lines in vitro, and exhibiting potent in vivo anti- tumor activity in xenograft studies.
SF2523 distinguishes from all reported PI3K inhibitors as it exhibits demonstrated anti- cancer stem cell and generation of oxidative stress activities. The biological, chemical, and computational profile of SF2523 have been published in J. Med. Chem. 2013, Vol. 56, pages 1922-1939 (doi: 10.1021/jm301522m).
Moreover, SF2523 also inhibits bromodomain proteins from binding to chromatin. This effect can inhibit the transcription of certain genes that are driving cancer. In particular, inhibition of the bromodomain protein BRD4 has been shown to block the transcription of the oncogene MYC (cMYC and MYCN). Others have shown that inhibition of PI3K enhances the degradation of the MYC protein. Thus, our dual PI3K/BRD4 inhibitors such as SF2523 provide a unique ability to maximally block MYC activity via two orthogonal mechanisms. These type of dual inhibitors should be particularly effective in MYC-driven tumor types such as CLL, multiple myeloma, medulloblastoma, and neuroblastoma. Screening of patients for such MYC-driven cancers should result in higher rates of responses in this patient population.
The relationship between PI3K and Bromodomain proteins and MYC are shown in the figure and legend below:
Figure Legend: SignalRx proposes to improve MYC inhibition by evaluating PI3K inhibitors (e.g. SF2523) that also inhibit BET (bromodomain proteins) such as BRD4. Facts supporting this approach are: 1) Many cell surface receptors (growth factor receptors, chemokine and cytokine receptors, integrins, etc.) pulse cancer signals through all PI-3K classes, 2) PI-3K inhibition enhances the degradation of MYC, and 3) BET (BRD4) inhibition decreases the transcription of MYC. Conclusion: Dual inhibition of PI3K/BET by SF2523 (red box) or other SignalRx compounds will augment potency against MYC driven malignancies.
The kinase and bromodomain protein (BRD4) inhibition profile of SF2523 shown below demonstrates good target potency:
PI3K-δ isoform is a great potential target for blood cancers with several delta-selective PI3K inhibitors in clinical development with GS-1101 leading the group. New information has just revealed the important role of delta-PI3K in breaking the regulatory T-cell-mediated immune tolerance to cancer (Vanhaesebroeck et al. Nature, 2014). It is not known if this will also be the case for pan-PI3K inhibitors which also inhibit delta-PI3K but not as selectively.
SignalRx’s compound SF2535 is a selective and potent delta isoform PI3K inhibitor with a similar profile to GS-1101 (CAL-101). Additionally, SF2535 also inhibits bromodomain proteins becoming the first-in-class dual inhibitor of its kind. The table below shows a much more delta-selective PI3K profile of SF2535 vs GS-1101 including a better BRD4 inhibitory attribute:
SF2535 is undergoing additional preclinical studies aimed at understanding any advantages or differences compared to the pan-PI3K/BRD4 inhibitor SF2523.
Summary of the problem:
The MAPK and PI3K cell signaling pathways play significant roles in cancerogenesis. These two pathways are activated upstream from cell surface receptors and, as in many cases, their signal traffic cross over from the MAPK pathway to the PI3K pathway and vice versa. The impact of such crossover pathways (i.e., feedback loops) in a cancer cell generally results in anticancer therapeutic resistance ultimately rendering monotherapeutic approaches limited or useless.
As a result of this compelling biological understanding, there are now 5 examples of combining PI3K inhibitor drugs and MEK inhibitor drugs in clinical trials even though neither agent has received FDA approval as monotherapeutic anticancer agents. This combining of experimental agents at such an early stage is unprecedented and highlights the desperate desire to inhibit both pathways for maximum efficacy. However, as in any combination clinical trial, this 2-drug approach suffers from:
1) High cost of developing two agents.
2) High drug costs ultimately for patients.
3) Doubling of the developmental risk of failure.
4) Difficult early clinical development with complex multiple combinations
The CRIMP platform has delivered SF2626, a next-generation dual pathway inhibitor that inhibits both MEK and PI3K at single digit micromolar IC50 values, thought to be the next level in the cancer target evolution.
HDAC Background: HDACs are enzymes that work by diminishing the amount of histone acetylation which leads to chromatin structure changes facilitating gene specific repression of transcription. HDACs form multiprotein complexes that ultimately participate in DNA-binding driven processes involved in cell-cycle progression and apoptosis. Dysregulation of HDACs-participating processes can lead to cancer development and/or progression.
PI3K/HDAC Dual Inhibition Background: There is ample evidence that HDAC inhibitors can impact several signaling networks including the PI3K pathway and there is growing evidence that combinations with other agents may provide significantly improved efficacy. This synergy has been demonstrated by others specifically with PI3K inhibitors and HDAC inhibitors both in vitro and in vivo in head and neck squamous cell carcinoma, non-small cell lung cancer, and endometrial carcinoma.
SignalRx has developed SF2558HA, a dual PI3K/HDAC inhibitor with favorable attributes such as: 1) low molecular weight; 2) high brain barrier and solid tumor penetrability; 3) inhibition of selected additional kinases (mTOR, PIM, DNA-PK); 4) generate oxidative stress; and 5) inhibit cellular efflux pumps. This compound is in early stage preclinical studies.
Using the CRIMP methodology we are investigating other combinations of targeted agents found to be synthetically lethal to cancer cells or cancer stem cells but not toxic to normal cells. These combinations are not yet disclosed.