Rebecca Kirk

Bio: Rebecca Kirk is an academic researcher. The author has contributed to research in topics: Breast cancer & Cancer. The author has an hindex of 7, co-authored 92 publications receiving 490 citations.

High drug attrition rates--where are we going wrong?

Abstract: Drug attrition rates for cancer are much higher than in other therapeutic areas. Only 5% of agents that have anticancer activity in pre clinical develop ment are licensed after demonstrating sufficient efficacy in phase III testing, which is much lower than, for example, 20% for cardiovascular disease. To compound this issue, many new cancer agents are being withdrawn, suspended or discontinued. Figure 1 illustrates that this trend is extremely prevalent for VEGF inhibitors although less so for drugs targeting Aurora B kinase and some targeted therapies. The reasons for this high attrition rate are complex; however, several articles in this issue provide insights into why this is occurring. In essence, the pre clinical strategies to evaluate novel agents are sub optimal, and identifying the correct target using appropriate preclinical models will be critical to prevent further drug failures. Ian Tannock and coauthors discuss the limitations of preclinical models for drug assessment. A key drawback of animal models is that they do not represent the primary tumors from which they are derived in terms of tumor heterogeneity and the mechanisms of drug resistance. Xenograft models lack the broad molecular transformation events that occur in human tumors. Furthermore, since the stromal component of the tumor is not human the effects of the microenvironment on drug response are often not reflective of the primary tumor. Importantly, the growth rates of human-derived xenografts are considerably more rapid than primary tumors and, as a result, are much more likely to respond to anti proliferative agents. Testing of antiproliferative drugs in animal models might provide a false indication of the potential efficacy of a drug. Also, the immune system in such animal models is compromised, hindering the testing of immunomodulatory agents. Genetically-engineered mouse models circumvent some of these limitations as they are immune competent but they still suffer from having rodent-derived stroma. Getting the target right is a crucial aspect of drug development. Komlodi-Pasztor and coauthors discuss why mitosis-specific agents—including those that target aurora kinases and polo-like kinases—have limited success in the clinic, especially compared with micro tubule-targeting agents (MTAs) that have proved success ful despite both classes of agent having the same putative target. A closer examination of the mechanisms of action of MTAs reveals that they exert their effects not on mitosis but predominantly via interphase cellular mechanisms and microtubules, which are present in both mitotic and non-mitotic cells. Most human tumors divide slowly, and so mitosis is rare or absent; therefore, it is an unlikely target for MTAs. The doubling times in humans means that cancer cells multiply at similar rates to bone-marrow cells, explaining why agents that target mitosis also cause neutropenia and myelosuppression. Komlodi-Pasztor et al. state that although mitotic kinases produced disappointing results in the clinic, the drugs were well designed and had a valid target—that is mitosis —but unfortunately their efficacy was at the expense of high toxic effects and the fact that not many of the tumor cells were ‘druggable’ because the target was transient. However, agents that target mitotic kinases have a lower attrition rate compared with other (non-targeted) cancer drugs (Figure 1), which might be because the trial data are not mature but this is promising nevertheless. These findings emphasize the importance of identifying and validating the target and understanding resistance mechanisms. Interestingly, regarding correct target identification, the HER2-targeting agent trastuzumab may have limited benefit in patients with very high levels of HER2, as indicated by an inferior recurrence-free survival compared with patients who express more moderate levels of the target (Joensuu, H. et al. Ann. Oncol. doi:10.1093/annonc/ mdq710). This might explain why some patients with HER2-positive tumors do not respond to trastuzumab, as this agent might affect other HER family members or compromise downstream signaling effects if HER2 levels are extremely high. Ebos and Kerbel postulate why the success of antiangiogenic agents in the metastatic setting has not been mirrored in the adjuvant setting. They suggest the nature of disease progression following antiangiogenic therapy might be distinct to that seen with cytotoxics. Evidence from preclinical studies indicates that in certain situ ations antiangiogenics may increase invasiveness and metastatic potential. In orthotopically implanted mouse tumors, an increase in metastasis and shortened survival was seen when mice were treated with an anti-VEGF therapy before metastasis was induced—thus short-term treatment with antiangiogenic drugs may influence micrometastatic disease, which has critical implications for the preclinical models used in drug development. It is possible that if VEGF suppression is not sustained this could lead to a ‘rebound’ effect in terms of tumor growth. This may explain the results from the recent AVANT trial, in which a positive effect on progression-free High drug attrition rates—where are we going wrong?

Genetics: Personalized medicine and tumour heterogeneity

Abstract: A team of what Cancer Research UK have described as “some of the UK's most exciting young researchers” have performed a careful genetic analysis of tumours and shown that “no two samples from the same patient were genetically identical.” This finding has obvious implications for personalized medicine.

Tumour evolution: Evidence points to the existence of cancer stem cells.

Abstract: Currently, in solid tumours there are two models for tumour proliferation. First, the classic theory in which all cancer cells have proliferative potential and can replicate and contribute to the expansion of the tumour. Second, the cancer stem cell (CSC) hypothesis suggests that there is a hierarchy of cell proliferation with a distinct population of cancer cells at the top of the pyramid. Now, two studies published in Nature and one in Science have provided evidence for the presence of CSCs in solid tumours for the first time in models of unperturbed tumour growth.

Risk factors. CD8+:FOXP3+ cell ratio is a novel survival marker for colorectal cancer.

Abstract: the tumor infiltrating CD8+:FoXP3+ cell ratio may be a predictive marker of diseasefree survival in patients with colorectal cancer, according to a recent study. a team at Kyushu university, Japan analyzed the relationships of intratumoral CD8+ t cells and FoXP3+ cells with traditional pathological measurements for surgically resected specimens from 95 patients with primary colorectal cancer. the researchers did not detect any correlation between the number of intratumoral CD8+ t cells with any pathological parameter; however, lymph-node metastasis was significantly associated with the number of intratumoral FoXP3+ cells. Furthermore, suzuki noted that “the ratio of the number of [intratumoral] CD8+ t cells to the number of [intratumoral] FoXP3+ cells (itCD8+:itFoXP3+ cell ratio) was negatively correlated with pathological stages.” although the number of CD8+ t cells and FoXP3+ cells was not associated with Risk factoRs

Drug development: Raise standards for preclinical cancer research

Abstract: C. Glenn Begley and Lee M. Ellis propose how methods, publications and incentives must change if patients are to benefit.

Advances in establishment and analysis of three-dimensional tumor spheroid-based functional assays for target validation and drug evaluation

Abstract: There is overwhelming evidence that in vitro three-dimensional tumor cell cultures more accurately reflect the complex in vivo microenvironment than simple two-dimensional cell monolayers, not least with respect to gene expression profiles, signaling pathway activity and drug sensitivity. However, most currently available three-dimensional techniques are time consuming and/or lack reproducibility; thus standardized and rapid protocols are urgently needed. To address this requirement, we have developed a versatile toolkit of reproducible three-dimensional tumor spheroid models for dynamic, automated, quantitative imaging and analysis that are compatible with routine high-throughput preclinical studies. Not only do these microplate methods measure three-dimensional tumor growth, but they have also been significantly enhanced to facilitate a range of functional assays exemplifying additional key hallmarks of cancer, namely cell motility and matrix invasion. Moreover, mutual tissue invasion and angiogenesis is accommodated by coculturing tumor spheroids with murine embryoid bodies within which angiogenic differentiation occurs. Highly malignant human tumor cells were selected to exemplify therapeutic effects of three specific molecularly-targeted agents: PI-103 (phosphatidylinositol-3-kinase (PI3K)-mammalian target of rapamycin (mTOR) inhibitor), 17-N-allylamino-17-demethoxygeldanamycin (17-AAG) (heat shock protein 90 (HSP90) inhibitor) and CCT130234 (in-house phospholipase C (PLC)γ inhibitor). Fully automated analysis using a Celigo cytometer was validated for tumor spheroid growth and invasion against standard image analysis techniques, with excellent reproducibility and significantly increased throughput. In addition, we discovered key differential sensitivities to targeted agents between two-dimensional and three-dimensional cultures, and also demonstrated enhanced potency of some agents against cell migration/invasion compared with proliferation, suggesting their preferential utility in metastatic disease. We have established and validated a suite of highly reproducible tumor microplate three-dimensional functional assays to enhance the biological relevance of early preclinical cancer studies. We believe these assays will increase the translational predictive value of in vitro drug evaluation studies and reduce the need for in vivo studies by more effective triaging of compounds.

Three-Dimensional Cell Culture: A Breakthrough in Vivo

Abstract: Cell culture is an important tool for biological research. Two-dimensional cell culture has been used for some time now, but growing cells in flat layers on plastic surfaces does not accurately model the in vivo state. As compared to the two-dimensional case, the three-dimensional (3D) cell culture allows biological cells to grow or interact with their surroundings in all three dimensions thanks to an artificial environment. Cells grown in a 3D model have proven to be more physiologically relevant and showed improvements in several studies of biological mechanisms like: cell number monitoring, viability, morphology, proliferation, differentiation, response to stimuli, migration and invasion of tumor cells into surrounding tissues, angiogenesis stimulation and immune system evasion, drug metabolism, gene expression and protein synthesis, general cell function and in vivo relevance. 3D culture models succeed thanks to technological advances, including materials science, cell biology and bioreactor design.

Cardiac angiogenic imbalance leads to peripartum cardiomyopathy

Abstract: Peripartum cardiomyopathy (PPCM) is an often fatal disease that affects pregnant women who are near delivery, and it occurs more frequently in women with pre-eclampsia and/or multiple gestation. The aetiology of PPCM, and why it is associated with pre-eclampsia, remain unknown. Here we show that PPCM is associated with a systemic angiogenic imbalance, accentuated by pre-eclampsia. Mice that lack cardiac PGC-1α, a powerful regulator of angiogenesis, develop profound PPCM. Importantly, the PPCM is entirely rescued by pro-angiogenic therapies. In humans, the placenta in late gestation secretes VEGF inhibitors like soluble FLT1 (sFLT1), and this is accentuated by multiple gestation and pre-eclampsia. This anti-angiogenic environment is accompanied by subclinical cardiac dysfunction, the extent of which correlates with circulating levels of sFLT1. Exogenous sFLT1 alone caused diastolic dysfunction in wild-type mice, and profound systolic dysfunction in mice lacking cardiac PGC-1α. Finally, plasma samples from women with PPCM contained abnormally high levels of sFLT1. These data indicate that PPCM is mainly a vascular disease, caused by excess anti-angiogenic signalling in the peripartum period. The data also explain how late pregnancy poses a threat to cardiac homeostasis, and why pre-eclampsia and multiple gestation are important risk factors for the development of PPCM.

Chemistry and biochemistry of 13C hyperpolarized magnetic resonance using dynamic nuclear polarization.

Abstract: The study of transient chemical phenomena by conventional NMR has proved elusive, particularly for non-1H nuclei. For 13C, hyperpolarization using the dynamic nuclear polarization (DNP) technique has emerged as a powerful means to improve SNR. The recent development of rapid dissolution DNP methods has facilitated previously impossible in vitro and in vivo study of small molecules. This review presents the basics of the DNP technique, identification of appropriate DNP substrates, and approaches to increase hyperpolarized signal lifetimes. Also addressed are the biochemical events to which DNP-NMR has been applied, with descriptions of several probes that have met with in vivo success.