So What Happened to the PARP Inhibitors in Breast Cancer Anyway? ASCO 2011

Many of you may recall that we described our studies with the small molecules BSI201 (iniparib) and AZD2281 (olaparib) (Nagourney, et al. ASCO 2011). Based upon the exciting Phase II data reported by Dr. Joyce O’Shaughnessy, first at the ASCO meeting, then in the NEJM, describing the remarkable efficacy of BSI201 (iniparib) combined with carboplatin and gemcitabine in triple negative breast cancer (TNBC), we initiated a study of both iniparib and olaparib in human breast cancer specimens. Our results were reported at the American Society of Clinical Oncology meeting.

Despite the enthusiasm that surrounded Dr. O’Shaughnessy’s initial observations, the confirmatory clinical trial using iniparib combined with carboplatin and gemcitabine, then compared with carboplatin and gemcitabine did not achieve statistical significance. That is, the trial was negative and the combo of inabirib with carboplatin plus gemcitabine was not proven superior.

So, what happened? Quite a few things.

It turned out that BSI201, a member of the benzamine chemical family, at physiological concentrations achievable in humans is not a PARP inhibitor. This, in retrospect, should have been obvious because a full-dose PARP inhibitor, plus a potent combination of carboplatin plus gemcitabine would not likely be tolerable if PARP inhibition were achieved.

Second, the patients receiving the drug are probably not a homogeneous population. That is, some TNBC patients may be similar to the BRCA patients, while others may not have the DNA repair deficiencies associated with PARP inhibitor response.

Finally, it was our group that originally reported the carboplatin plus gemcitabine combination in breast cancer, as a split-dose doublet in 2008 (Nagourney, Clin Breast Cancer Research, 2008). We observed, in that original clinical trial, that even a lower starting dose of gemcitabine (i.e. 800mg/ml2 vs. the O’Shaughnessy 1000 mg/m2) resulted in significant toxicity and in our concluding comments in that paper, we suggested 600mg/ml2. At 1000 mg/m2, Dr. O’Shaughnessy’s trial nearly doubled our recommended dose in this patient population.

While our abstract did not receive the fanfare of the clinical trial, it was, in fact, remarkably prescient. We, like other investigators, entered into our original studies of these molecules believing iniparib to be a PARP inhibitor. To our surprise, and, in retrospect, to our credit, a direct comparison of olaparib (AZD2281) to inapaprib (BSI201) revealed no correlation. We described this in our abstract, “Of interest, BSI201 & AZD2281 activity did not correlate in parallel analyses (R = 0.07, P > 0.5).”  Thus, our human tumor primary culture analysis scooped the ASCO investigators. Unfortunately, it appears they weren’t listening.

So, what have we learned? First, we’ve learned that iniparib is not a true PARP inhibitor.

Second, we learned that the combination of platins plus gemcitabine in breast cancer is synergistic, highly active and can be toxic (particularly at the doses chosen for this trial).

Finally, we learned that TNBC, indeed all breast cancers, even more to the point, all cancers in general, are heterogeneous. That is precisely why the use of human tumor primary culture analyses are so instructive and should be incorporated into clinical trials for these and other targeted agents.

Can PARP Inhibitors be Tested Using the EVA-PCD Assay?

Poly ADP ribose polymerase (PARP) is a nuclear enzyme associated with response to DNA damage. Following single strand DNA breaks, the enzyme attaches a backbone of ADP and ribose that serves to initiate DNA repair. Certain classes of chemotherapeutics, specifically alkylating agents, can induce injury that results in extensive poly ADP ribosylation resulting in the exhaustion of intercellular pools of NAD and ATP ultimately leading to cell death.

Although PARP inhibitors have recently entered the clinical cancer literature mostly relating to the treatment of BRCA+ and triple negative patients, neither PARP nor PARP inhibitors are new to the cancer researcher community.

Our group first became interested following a 1988 study by Distelhorst from Case Western Reserve (Distelhorst CW, Blood 1988 Oct;72(4):1305-09) that described a mechanism of cell death that correlated with our work in childhood leukemia. Previously, investigators at Scripps Clinic had described PARP’s role in response to 2CDA (Seto, S., et al. J Clin. Invest. 1985 Feb;75(2):377-83). We have studied small molecule inhibitors of PARP for many years, and more recently, we have expanded these investigations to include BSI201 (iniparib) and AZD2281 (olaparib). Both of which are undergoing clinical investigations. We will be reporting our findings with these PARP inhibitors at the 2011 ASCO meeting (Nagourney, R., et al Proceedings Amer Soc Clin Oncol. 2011).

PARP inhibitors are easily studied and provide interesting signals in the tissue studied. We see activity in BRCA+ patients and some triple negative breast cancers. We have also identified synergy with other classes of drugs. The compounds are a welcome addition to our cancer therapy armamentarium and continue to be actively studied in the EVA-PCD platform.

Of interest is the recent failure of the iniparib plus Carboplatin & gemcitabine Phase III trial to meet progression-free and overall survival goals in triple negative breast cancer patients (Zacks Investment Research on January 31, 2011). This failure may reflect the need to apply predictive methodologies to select candidates for these drugs, similar to our successful work with other classes of compounds.

Why do People get Cancer?

While there are a lot of reasons why people develop cancer, there is a growing recognition that a subset of patients carries genetic predispositions for the disease. Some of these genetic syndromes result in childhood cancers like the retinoblastoma gene or mutations in P53. These abnormalities are so profound that virtually all patients develop aggressive cancers at an early age. However, there is a second group of genetically driven cancers that are being encountered in young and middle aged adult patients. One of the best described is the ovarian/breast cancer syndrome associated with the BRCA 1 and 2 genes. Another group of patients carry a DNA repair deficiency known as mismatch repair or Lynch Syndrome.

Not unlike the BRCA patients, people with mismatch repair have an inability to respond to DNA damage. This failure leads to mutational events that, over the course of a lifetime, can result in cancer. We now know that the BRCA genes may provide therapeutic opportunities as the new class of drugs known as PARP inhibitors can target them. What we are now learning is that the Lynch Syndrome patients may have a similar attribute that can, in some circumstances, render them “hypersensitive” to chemotherapeutics. One such patient has been under my care for the last two years.

This charming 43-year-old patient presented with cancer of the uterus. She was managed by a gynecologic oncology service and received a combination of surgery, radiation and chemotherapy. One year later, she revealed recurrent disease in the right, lower abdomen with involvement in the liver. Impending bowel obstruction lead to surgical exploration, providing my laboratory with tissue for analysis. When I first received the tissue specimen, I was expecting recurrent uterine cancer, the same diagnosis for which she had been treated the year earlier. But, to my surprise, the patient was actually diagnosed with colon cancer. This triggered an analysis of her mismatch repair gene and provided confirmation of Lynch Syndrome.

What I found amazing was that this patient’s colon cancer was sensitive to a two-drug combination that I had never in my career administered for colon cancer. Indeed, in my published work I had consistently identified colon cancer as a bad target for this doublet. Yet, this patient’s tumor was unequivocally sensitive to the combination. Her response was as prompt as it was dramatic — a complete remission within a scant few months. And then, in follow up, her PET/CT revealed a small focus of abnormality, seemingly associated with the colon. With a negative colonoscopy, we waited an additional several months and repeated the study. This time, it was even more evident; there was clearly an abnormality in the left pelvis.

A biopsy provided an unexpected finding. It was cancer, but it wasn’t colon cancer. The patient’s original uterine cancer from two years earlier had recurred, most likely as a residual vestige of tumor from an incomplete resection two years before. The drug response profile was distinctly different, but highly consistent with a profile one might find in a patient with mismatch repair. As we prepared to treat the patient, she developed gastrointestinal bleeding, a workup for which confirmed erosion by the uterine cancer into the bowel wall. We decided to use our findings to treat the patient and initiated a three-drug combination. The patient’s tolerance was excellent and gastrointestinal bleeding stopped immediately.

She is now receiving additional courses of therapy and will be evaluated for response in the coming months. While it is too early to know how well she’ll respond, we are optimistic regarding her outcome. Among the most interesting feature of this and related cases is the fact that the genetic mutation that caused her cancer may be the same genetic mutation that makes it possible for us to treat her.

Emerging Therapies in Breast Cancer: a Focus on Triple Negative Disease

As our understanding of breast cancer biology continues to advance, this disease has come to be understood as many different diseases. Original categorizations based on histology lead to lobular versus ductal subtypes. Thereafter, recognition of estrogen and progesterone status, and finally HER2 status provided further subcategorizations. Over the past decade, molecular subtypes have characterized this disease into a series of signatures characterized by luminal, basal and other groupings with distinct prognoses. Within the context of these categories, the triple negative breast cancers have emerged as an important target. These patients whose tumors do not mark for estrogen, progesterone, or HER2 on immunohistochemical or FISH analyses, appear to carry features that segregate them into a BRCA1-like biology. This is of great interest clinically for it offers the opportunity to treat these patients with drugs found active in the BRCA mutant populations. Among the most active drugs in these patients are the PARP inhibitors. The excellent results with PARP inhibitors and BRCA mutants have been followed by striking response and survival data combining PARP inhibitors with carbo-platinum and gemcitabine. PARP inhibitors by inhibiting DNA damage response can enhance the effects of ionizing radiation, mustard alkylators, topoisomerase inhibitors, platins, and intercalating agents. We have explored the biology of PARP inhibitors in breast and other cancers. In these investigations, our lab to applies the EVA-PCD™ platform to understand how PARP inhibitors enhance the effects of drugs and drug combinations. To date, we have observed good activity for the PARP inhibitors as single agents in BRCA1 positive patients, and in some triple negative patients. More interesting, will be the results combining the PARP inhibitors with mustard alkylators, platins, and drug combinations to optimize PARP inhibitor combinations. This work is ongoing in triple negative and BRCA positive patients as well as other tumor types where the PARP inhibitors may prove useful in the future.