Monthly Archives: December 2012

SABCS mini-series post 2: What’s new and upcoming in triple negative breast cancer?

In this second overview of research presented at SABCS, I thought I would focus on the excitement building with regard to options in triple negative breast cancer (that is tumors that don’t express the estrogen receptor, progesterone receptor or HER2).

In the past, triple negative disease has been defined in terms of what proteins are NOT expressed, but at this meeting, we heard from Dr Jennifer Pietenpol how TNBC is actually a heterogenous disease with 7 subtypes based on a large scale gene expression analysis of more than 3000 tumors from 21 worldwide data sets (not a trivial bioinformatics effort!).  The major take home talk from her fabulous talk is that most of these subtypes have particular targets that may be feasible for targeting. As an example, about 11% of TNBCs actually express the Androgen Receptor (called the Luminal Androgen Receptor subtype), which is known to be a key growth regulator in prostate cancer, but until now has not received much attention in other tumors.  The availability of AR antagonists such as bicalutamide means that now it is possible to target AR in breast cancers as well, although more detailed characterization of its function in breast cancer is still needed.  As such, there will shortly be a trial available at Vanderbilt, using bicalutamide in combination with a PI3K inhibitor (because this subtype is highly enriched for PIK3Ca mutations – in TCGA dataset 50% of the LAR subtype had PIK3Ca mutations vs 3% in TNBCs as a whole).  In case you are interested in reading more, the majority of Dr Pietenpol’s data was recently published in the Journal of Clinical Investigation (Lehmann et al, see references below).

Another interesting talk in the TNBC session on Tuesday was by Trey Westbrook, a young professor at Baylor College of Medicine. His research is focused on using genetic screens to uncover new targets. This functional approach is complementary to many of the large scale genomic screens such as the TCGA and ICGC that are being performed since targets that inhibit tumorigenesis that overlap with gene-level changes (amplification/mutation) are more likely to be true drivers vs passenger changes. In addition, targets that are validated functionally allow us to narrow down a large number of potential molecules for further study, since drug development and even basic research to understand mechanism is quite expensive.

So with that background, I will describe some of Dr Westbrook’s findings from his basic screen using a shRNA library transfected into normal mammary cells that have been immortalized with telomerase and SV40.  The goal was to figure out which shRNAs induce transformation of these immortalized cells, which is tested using a standard anchorage-independent growth assay.  A total of 42 new tumor suppressors were uncovered in this screen, but he only presented data regarding one of them,  that is PTPN12.  PTPN12 is a phosphatase that is mutated in about 5% of TNBCs but when analyzed further is found to be down-regulated by other (unknown) mechanisms in about 60% of TNBC tumors. Next the lab asked what does PTPN12 do? Phosphatases act in opposition to kinases that add phosphate groups to other proteins ie they remove these phosphates, which usually (but not always) turns off a protein.

Figure 1: Network of proteins regulated by PTPN12

Figure 1: Network of proteins regulated by PTPN12

To figure out which proteins PTPN12 acts on, they did an elegant proteomic experiment using SILAC and compared the phospho-tyrosine proteome in cells with high PTPN12 levels and shRNA-depleted cells. What they found was that PTPN12 regulates a whole network of tyrosine kinase receptors including EGFR, HER2 and PDGFb, and that collectively these pathways drive tumorigenesis (see figure 1). This is exciting because there are currently available drugs against many of these pathways, so there is renewed enthusiasm in testing novel combinations of TKIs in this biomarker-defined subset (that is PTPN12 low). As a proof of principle preclinical study in xenograft tumors in mice, a combination of crizotinib and sunitinib to target cMet and PDGFRb (respectively) was tested. Individually each agent showed only very minimal tumor growth delay, however in combination tumor growth was halted, and the mice lived statistically longer (see key tumor growth rate in figure 2).

Figure 2: Tumor growth rate in mice treated with crizotinib and/or sunitinib

Figure 2: Tumor growth rate in mice treated with crizotinib and/or sunitinib

Once again, some of this work has been published recently in Cell, although this specific pharmacological combination was not presented in the publication (Sun T et al, see references)

References:

Lehmann B et al, “Identification of human triple-negative breast cancer subtypes and preclinical models for selection of targeted therapies” JCI 2011

Sun T et al, “Activation of multiple proto-oncogenic tyrosine kinases in breast cancer via loss of the PTPN12 phosphatase” Cell 2011

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SABCS Mini-series post 1: Pfizer CDK4/6 inhibitor is a #win

I just returned from my first San Antonio Breast Cancer Symposium which was an information-packed week of talks, posters, and meeting interesting people.  I had a blast, learned a ton, and made a valuable connection at a company for a future study.  The next few posts on my blog are going to cover the biggest news items as well as what I personally found the most interesting at the meeting.  But outside of these topics, if you have any questions, feel free to comment and maybe I’ll add a post or just get back to you on it. The online resources available to the attendees of the meeting are vast so even if I didn’t attend that session, I should be able to get an answer for you from them or my twitter colleagues who attended.

I will kick off this short series of posts, with what I thought was the most promising new therapeutic advance, and this is the latest analysis of the trial of Pfizer’s CDK4/6 inhibitor, PD-0332991.   When I was reading up on drugs for the book chapter I just wrote on cell cycle targeted therapies, apart from my own drug of interest, I was most excited about this compound.  And indeed, the clinical results of a randomized phase 1/2 study presented by Richard Finn were stunning! The progression-free survival curve presented is shown in figure 1. “LET” stands for letrozole, a non-steroidal aromatase inhibitor, an FDA approved drug that is currently used in post-menopausal patients with hormone-sensitive tumors.

Progression-free survival curves for PD-0332991

Figure 1: Progression-free survival curves for letrozole vs PD+letrozole in postmenopausal ER+ patients.

To elaborate on this study, which was a worldwide multi-center trial of postmenopausal ER+, HER2- patients based on the preclinical work that highlighted that targeting CDK4/6 only works in the context of cells with intact G1 checkpoint (ie wild-type Rb). In case you are not familiar with the cell cycle, figure 2 has a schematic of the cell cycle and proteins involved in each phase.

Cell cycle schematic

Figure 2: Cell cycle schematic

Note that CDK4 and CDK6, targets of this drug,  function in complex with cyclin D1 to phosphorylate and inactivate Rb, which is a major cellular brake on cell proliferation. It would make sense then that targeting CDK4/6 in the context of tumors cells that have lost Rb would be ineffective. Since luminal (ER positive) breast cancers also frequently overexpress cyclin D1, Pfizer decided to focus on these patients, however they do have an additional 3 separate trials in lymphoma and myeloma, based on preclinical data in these models too.

This drug was incredibly well tolerated, with the majority of the AEs being grades 1 or 2. The only added toxicity over letrozole alone that was observed in more than a few patients was neutropenia, which was described by Dr Finn as “uncomplicated”, and expected for this class of compounds which do affect normal cells as well.

The results of the biomarker analysis was intruiging however for the basic scientists among us. Even though it was thought that cyclin D1 amplification or loss of p16 (which is a negative regulator of CDK4/6 activity) would be predictive biomarkers of sensitivity, and were used to enrich the phase 2 part of this trial for potential responders (after phase 1 found the maximum tolerated dose). However, upon biomarker analysis at the end of the study, they found that these proteins were no better than ER alone at predicting response.  Clearly, further work is necessary to find a biomarker of response (and/or resistance), even though the clinical benefit rate of 70% seen in this study was quite impressive. In conclusion though, based on these data (both preclinical and this study), a registration study is planned to start in 2013.

References (journal articles):

Preclinical breast cancer papers:

“PD 0332991, a selective cyclin D kinase 4/6 inhibitor, preferentially inhibits proliferation of luminal estrogen receptor-positive human breast cancer cell lines in vitro.” – Breast Cancer Res Treatment 2009, pubmed link

“Therapeutic response to CDK4/6 inhibition in breast cancer defined by ex vivo analyses of human tumors.” – Cell cycle 2012, pubmed link

Preclinical other tumors:

“A novel orally active small molecule potently induces G1 arrest in primary myeloma cells and prevents tumor growth by specific inhibition of cyclin-dependent kinase 4/6.” – Cancer Res 2006, pubmed link

“Pharmacologic inhibition of CDK4/6: mechanistic evidence for selective activity or acquired resistance in acute myeloid leukemia.” – Blood 2007, pubmed link

“Pharmacologic inhibition of cyclin-dependent kinases 4 and 6 arrests the growth of glioblastoma multiforme intracranial xenografts.” – Cancer Res 2010, pubmed link

“Selective CDK4/6 inhibition with tumor responses by PD0332991 in patients with mantle cell lymphoma” – Blood 2012, pubmed link

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