PARP Inhibitors For Breast, Ovarian And Other Cancers

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Some cell-based assay labs have explored the biology of PARP inhibitors, alone and in combination, in actual human tumor primary culture micro-speheroids (microclusters), in breast, ovarian and other cancers. In these investigations, the lab applies the functional profiling platform to understand how PARP inhibitors enhance the effects of drugs and drug combinations...

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The Sunday, April 3, 2011, experimental and molecular therapeutics poster session at the AACR 102nd annual meeting included Dr. Robert Nagourney's Rational Therapeutics presentation on signal transduction inhibitors. Using MEK/ERK and PI3K-MTOR inhibitors he explored the activities, synergies and possible clinical utilities of these novel compounds.

The findings were instructive. First, it saw a good signal for both compounds utilizing the Ex-vivo Analysis of Programmed Cell Death (EVA-PCD) platform (functional profiling). Second, it saw disease-specific activity for both compounds. For the MEK/ERK inhibitor, melanoma appeared to be a favored clinical target. This is highly consistent with expectations. After all, many melanomas carry mutations in the BRAF gene, and BRAF signals downstream to MEK/ERK. By blocking MEK/ERK, it appeared that his lab blocked a pathway fundamental to melanoma progression. Indeed, MEK/ERK inhibitors are currently under investigation for melanoma.

For PI3K inhibitors, the highest activity was observed in uterine cancers. This has interest, because uterine carcinomas are often associated with a mutation in the PTEN gene. PTEN is a phosphatase tumor suppressor that functions to block activation of the PI3K pathway. Thus, mutations in the tumor suppressor unleash PI3K signaling, driving tumors to grow and metastasize. Blocking PI3K provided a strong signal, indicating that this approach may be very active in tumors associated with these oncogenic events.

The third point of interest in the report was, perhaps, its most important. Specifically, that the lab can explore those diseases where MEK-ERK, PI3K and mTOR signaling are less established targets. Cancers of the lung, ovary, colon or breast all manifested profiles of interest. When they combined both pathway inhibitors in a process called horizontal inhibition, renal cell carcinoma popped up as the best target. These results, though exploratory, suggest a superior approach for drug development, allowing the lab to identify important leads much faster than the clinical trial process.

Source: Rational Therapeutics, Inc.

Poster from Rational Therapeutics Session at 2011 AACR Meeting

http://robertanagourney.wordpress.co...-aacr-meeting/

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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, according to Dr. Robert Nagourney, medical director at Rational Therapeutics, one of the pioneers of the functional profiling technique.

His 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 their 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). His group has studied small molecule inhibitors of PARP for many years, and more recently, they have expanded these investigations to include BSI201 (iniparib) and AZD2281 (olaparib). Both of which are undergoing clinical investigations. Nagourney will be reporting their 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. They have seen activity in BRCA+ patients and some triple negative breast cancers. They have also identified synergy with other classes of drugs. The compounds are a welcome addition to the cancer therapy armamentarium and continue to be actively studied in the cell-based functional profiling 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 Nagourney's successful work with other classes of compounds.

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The recently analyzed evolutionary history of the poly(ADP-ribose)polymerase (PARP) found these proteins in eukaryotes, a wide range of organisms - animals, plants, molds, fungi, algae and protozoa - whose cells contain complex structures enclosed within membranes. While PARP proteins can be found with any of these groups, they have been most extensively studied in mammals.

In these organisms, PARPs have key functions in DNA repair, genome integrity and epigenetic regulation. More recently it has been found that proteins within the PARP family have a broader range of functions that initially predicted.

Researchers used computers to identify 236 PARP proteins from 77 species across five of the six groups, and performed extensive phylogenetic analyses of the identified PARP regions.

PARPs are found in all eukaryotic groups for which sequence is available, but some individual lineages within groups have independently lost these genes. The PARP family can be subdivided into six branches or clades. Two of these clades were likely found in the last common eukaryotic ancestor. In addition, they have identified PARPs in organisms in which they have not previously been described.

Three main conclusions were drawn from the study:

First, the broad distribution and pattern of representation of PARP genes indicated to the researchers that the ancestor of all existing eukaryotes encoded proteins of this type.

Second, the ancestral PARP proteins had different functions and activities. One of these proteins likely functioned in DNA damage response.

Third, the diversity of the PARP family is larger than previously documented, suggesting as more eukaryotic genomes become available, this gene family will grow in both number and type.

Source: The study, "Evolutionary history of the poly(ADP-ribose) polymerase gene family in eukaryotes," was authored by Rebecca S. Lamb, PhD, an assistant professor of Molecular Genetics and Ohio State University colleagues Matteo Citarelli and Sachin Teotia and appeared in an issue of the journal BMC Evolutionary Biology. The work was supported by a grant from the Ohio Plant Biotechnology Consortium and by funds from the Ohio State University.

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Functional analysis of PARP inhibitors AZD 2281 and BSI-201 in human tumor primary cultures: A comparison of activity and examination of synergy with cytotoxic drugs.

Sub-category: DNA Repair and Apoptosis

Category: Developmental Therapeutics - Experimental Therapeutics

Meeting: 2011 ASCO Annual Meeting

Abstract No: e13599

Citation: J Clin Oncol 29: 2011 (suppl; abstr e13599)

Author(s): R. A. Nagourney, K. R. Kenyon, F. R. Francisco, P. J. Bernard, S. S. Evans; Rational Therapeutics, Long Beach, CA

Abstract:

Background:

Poly (ADP-ribose) polymerases (PARP) are activated in response to cellular injury. DNA damage from radiation and cytotoxic drugs results in the up-regulation of PARP 1/2, leading to base excision repair. PARP inhibition enhances chemotherapy and induces cell death by synthetic lethality in patients with deficient homologous repair (BRCA1/2 and ATM). PARP inhibitors in development include benzamides, phthalazinones and benzimidazoles. Our work with 3-aminobenzamide (3-AB) led to the study of BSI-201 and AZD 2281, in human tumor micro-spheroids, isolated from surgical specimens and cytologically (+) fluids.

Methods:

Delayed loss of membrane integrity, morphologic and metabolic measures of drug-induced programmed cell death (EVA/PCD) were applied in 45 human tumor specimens exposed to PARP inhibitors, alone and in combination with cytotoxics. Lethal concentrations (LC50) were interpolated from 5-point dose response curves. Synergy was assessed by median effect. Drug activity comparisons were performed by modified Z-score.

Results:

PARP inhibitors are active in human tumor micro-spheroids. Activities for AZD 2281 and BSI-201 are superior to 3AB favoring BRCA1/2 and triple-negative (TN) breast over wild type and ER/PR (+); (AZD avg LC50 12 vs. 60 ug/mL; BSI avg LC50 19 vs. 30 ug/mL). AZD2281 and BSI-201 reveal synergy with CDDP, CDDP and gemcitabine, and alkylators. Of interest, BSI-201 and AZD-2281 activity did not correlate in parallel analyses (Pearson Moment, r = 0.07, P > 0.5). A comparison of BSI-201 and AZD 2281 activity with CDDP or taxol, suggested correlation with CCDP but not with taxol.

Conclusions:

1) PARP inhibitors are active in human tumors favoring BRCA1/2 and TN breast. 2) Favorable interactions with DNA damaging agents are observed. 3) Activity profiles correlate more strongly with CDDP than taxol. 4) Direct comparisons suggest somewhat different activity profiles for BSI-201 vs. AZD-2281. 5) Individual activity/synergy profiles may provide opportunities for patient selection in the development of novel PARP combinations. Analyses in BRCA 1/2 and TN breast cancers are ongoing.

Supported by The Vanguard Cancer Foundation and The Nagourney Institute.