Today I’m delighted to announce that we have a guest post from Adam Bristol, Ph.D, who works for Aquilo Capital Management in San Francisco.  Adam helps to manage a life sciences investment fund, where they invest in new drug discovery and drug development companies.  He also told me that his scientific training was in neuroscience, but “I’ve become absolutely fascinated with oncology and virology since I left the lab.” This is wonderful to hear and his enthusiasm really comes across in his report below.

At the recent American Society of Hematology meeting I was a tad grumpy at not being able to attend one of the science sessions and a lively discussion followed in the comments.  Adam wrote that he was attending the session and kindly offered to share his notes with the PSB community, which I’m delighted to post forthwith…

Readers of the PharmaStrategy Blog will be familiar with a scientific session entitled, “DNA-Repair Pathways: Cancer Syndromes to Novel Therapies” held on Monday afternoon at the recent ASH meeting.  With an expanding list of PARP inhibitors advancing in the clinic for multiple tumor types, and a growing literature on the biology of DNA repair mechanisms, this session promised to be a terrific overview of an emerging treatment paradigm.  However, as noted in an earlier post here, the conference program stated that attendance was limited to “medical professionals” only, thereby excluding a significant portion of ASH attendees.  While I’m not a medical professional, I was permitted to attend the session, perhaps because I registered before the restriction was imposed.  Below is the published abstract for the session and my take-aways from a very engaging presentation and discussion, which was chaired by Drs. Alan D’Andrea (Dan-Farber Cancer Institute) and Michael Kastan (St. Jude Children’s Research Hospital).  As with all scribbled notes on oral presentations, mine required some decoding as I began my write-up.  Corrections and elaborations from readers are more than welcome!

Here is a quick synopsis of the session from ASH:

“Conventional anticancer therapy (chemotherapy and radiation) kills tumor cells by causing DNA damage. Tumors differ in their response to these agents, at least in part, through their variable levels of DNA-repair activity. Human tumor cells have six independent DNA-repair pathways, including base-excision repair (BER), nucleotide-excision repair (NER), homologous recombination (HR), mismatch repair (MMR), non-homologous endjoining (NHEJ), and translesion DNA synthesis (TLS). Here, we will discuss the six major DNA-repair pathways found in human tumors, the relevant inherited cancer syndromes, the available biomarkers for assessing these pathways, and the emerging class of drugs referred to as DNA-repair inhibitors. These inhibitors, including those that target PARP or the ATM protein kinase, block DNA-repair pathways and can enhance the sensitivity of tumor cells to conventional therapy. Dr. Alan D’Andrea will discuss the Fanconi anemia/BRCA pathway and its synthetic lethal relationship with other DNA-repair mechanisms. Pharmacologic modulation of this pathway has led to novel therapies for cancer and for bone marrow failure. Dr. Michael Kastan will review another critical DNA-damage-response pathway, the ATM-p53 pathway. This pathway presents opportunities for development of novel anticancer agents, including potential approaches for both radiosensitization and radiation- or chemo-protection. As with the Fanconi anemia/BRCA pathway, the concept of synthetic lethality may also apply to this signaling pathway. Thus, targeting these pathways could lead to preferential killing of tumor cells based on the genetic or microenvironmental abnormalities in the tumors.”

Dr. D’Andrea’s presentation led off the session. He first noted that cancer cells are often defective in a DNA repair pathway and that this deficiency alters sensitivity to DNA-damaging chemotherapeutic agents. This leads to the possibility that knowing the status of DNA repair pathways could aid in predicting tumor sensitivity to chemotherapy. He cited a few examples, such as loss of BRCA1 and sensitivie to PARP inhibitors, low levels of DNA excision repair protein ERCC-1 predictive of cisplatin sensitivity, and levels of DNA repair protein O6-methylguanine-DNA methyltransferase (MGMT) associated with temozolomide sensitivity/resistance.

D’Andrea then summarized a significant amount of work that he and his colleagues have conducted on Fanconi Anemia (FA), a rare disease resulting from defects in thirteen DNA repair proteins, specifically the DNA repair response to cross-linking agents. Consequently, FA patients are hypersensitive to mitomycin C (MMC) and other cross-linking agents.  Three of the thirteen FA genes, including BRCA2, are known to be susceptibility genes for breast cancer and ovarian cancer.  D’Andrea proposed that loss of ubiquitination of the D2 Fanconi protein (FANCD2) can be useful biomarker of tumor susceptibility to DNA damaging agents because this ubiquitination step is the results of a DNA-damage-induced assembling of eight FA proteins into a multi-protein complex that acts as a E3 ubiquitin ligase that mediates the repair process.  In other words, because knocking out any of the protein in this process causes FA, tumors with low or no ubiquitination of FANCD2 should be sensitive to MMC or cisplatin.  He cited data showing that 20% of ovarian cancer biopsies showed deficits in the FA pathway (likely due to silencing by epigenetic events, such as methylation).

Ovarian cancer was not the only tumor type with a deficient FA pathway:

  • 18% of breast cancers
  • 15% of NSCLC
  • 15% squamous cell head and neck cancers
  • 35% cervical cancers

Sporadic AML, including those with the complex karyotypes of secondary AML, also commonly harbor FA deficiencies.  Dr. D’Andrea noted that de-methylatation, and thus restoration of FA pathway DNA repair, can also occur, and that this could be mechanism of tumor resistance.  Thus, serial tracking of FANCD2 ubiquitination can be used to track resistance.

He next raised the question of whether the FA pathway is druggable to effectively sensitize tumors to chemo agents.  That is, can we inhibit the mono-ubiquitination of FANCD2?  D’Andrea briefly reviewed the results of a chemical screen that found three kinase inhibitors but, importantly, also bortezomib (Velcade), inhibited FANCD2 ubiquitination.  Subsequent in vitro data showed that bortezomib sensitized ovarian cancer cell lines to cisplatin, both when bortezomib was administered as a pre-treatment or when administered concurrently with cisplatin.  Based on these and other data, they are currently running a Phase 1b trial of bortezomib in combination with cisplatin.

Lastly, Dr. D’Andrea discussed the concept of synthetic lethality and PARP inhibitors. Inhibitors of PARP, which stands for poly (ADP-ribose) polymerase, work because, in breast cancer tumors, cells become reliant on a specific DNA repair pathway, specifically base-excision repair.  He called them “essentially homologous-repair deficient tumors”, meaning that breast cancer tumor have lost that DNA repair pathway.  The problem, he said, is that resistance to PARP inhibitors has already been observed and that secondary tumors can be error-free in their repair.  At present, the rules for tumor specificity/sensitivity to PARP inhibitors is not well understood.  The question of resistance to PARP inhibitors resurfaced in the Q&A, at which time Dr. D’Andrea explained that the mechanism is not completely known but that it may be due to somatic reversion, i.e., mutant allele reverting to WT-like allele, thus reinstating the HR repair pathway.

Dr. Kastan gave the second talk of the session, beginning his remarks with a particularly sobering reminder: DNA is damaged in many ways, for example:

  • on purpose, such as when we are intentionally exposed to X-rays
  • on accident, such as when we are subjected to excessive sun exposure
  • unavoidably, such as the damage due to production of reactive oxygen species, a natural byproduct of cellular metabolism and ageing.

He cited the incredible estimate that our cells deal with ~10,000 instances of DNA damage per cell per day!  Exploration of DNA repair pathways is therefore relevant to the areas of metabolism, apoptosis and autophagy, noting specifically that interest in autophagy is really exploding at present.  Autophagy is a process of cellular self-destruction in which organelles and macromolecules are targeted and degraded by the lysosome.  In general, apoptosis and autophagy are important brakes on tumorigenesis and deficiencies in these processes, such as by alteration in tumor suppressor genes, are common proliferative strategies.

He noted that Arf/p53 tumor suppressor pathway is responsive to DNA damage (among other signals), in part through activation by Ataxia-Telangiectasia-mutated (ATM) protein kinase.  For example, ionizing irradiation activates ATM as does chemotherapy. Similarly, mutations in ATM (as in ataxia-telangiectasia, the kinase’s namesake) and a related kinase, ATM- and Rad3-related (ATR) kinase, have been shown to render mice and humans hypersensitive to ionizing radiation.  Thus, in theory, an ATM inhibitor could be a chemosensitizer.  It turns out that caffeine is an ATM/ATR inhibitor, but with poor potency.

Dr. Kastan and his colleagues have collaborated with Pfizer scientists to identify novel small molecule ATM inhibitors.   They’ve published on first generation compounds, and are now onto next generation molecules, specifically to explore the SAR, increase bioavailability, and perform siRNA screens for other related targets.  Achieving ATM/ATR selectivity would be challenging, however, as the enzymes are closely related.

On the other hand, activation of ATM kinase and subsequent induction of Arf/p53 could serve as a tumor prevention strategy.  Chloroquine, a anti-malarial drug originally discovered in the 1930’s, activates the ATM kinase and induces Arf/p53 without damaging DNA.  In a mouse model of ATM-deficient, Myc-driven tumors (a model of Burkitt lymphoma), Kastan showed (in Maclean et al., JCI, 2008) that weekly, low dose choloquine could prevent spontaneous de novo lymphomas.  He also cited clinical epidemiological data in which incidence of lymphoma in Tanzania was reduced by 75% during a trial of chlorquine as a treatment for malaria (the investigators were testing the hypothesis that malaria was a causative factor in development of Burkitt lymphoma, thus the collection of the incidence data).  Strikingly, the incidence rate returned to the pre-trial baseline within two years of the study’s completion.

Dr. Kastan also noted that a paper in Science in 1999 described pifithrin-alpha has capable of inhibiting p53 activation and preventing chemotherapy-induced apoptosis (a potential strategy to prevent an unwanted side effect in normal tissues), but the exact mechanism is still unclear.   He also discussed the use of phosphorylated ATM is a potential biomarker of ROS status.

In my opinion, both speakers presented compelling case studies of bench-to-bedside translational science; in each case, an understanding of the cell signaling pathways, their alteration and involvement in tumorigenesis and progression yielded therapeutic targets and novel treatment strategies.  I hope we see additional advances as this field progresses.

References: Komarov PG, Komarova EA, Kondratov RV, Christov-Tselkov K, Coon JS, Chernov MV, & Gudkov AV (1999). A chemical inhibitor of p53 that protects mice from the side effects of cancer therapy. Science (New York, N.Y.), 285 (5434), 1733-7 PMID: 10481009

Maclean, K., Dorsey, F., Cleveland, J., & Kastan, M. (2008). Targeting lysosomal degradation induces p53-dependent cell death and prevents cancer in mouse models of lymphomagenesis Journal of Clinical Investigation, 118 (1), 79-88 DOI: 10.1172/JCI33700