Pediatric oncology

Targeted drug discovery for pediatric leukemia

Despite dramatic advances in the treatment of pediatric leukemia over the past 50 years, there remain subsets of patients who respond poorly to treatment. Many of the high-risk cases of childhood leukemia with the poorest prognosis have been found to harbor specific genetic signatures, often resulting from chromosomal rearrangements. With increased understanding of the genetic and epigenetic makeup of high-risk pediatric leukemia has come the opportunity to develop targeted therapies that promise to be both more effective and less toxic than current chemotherapy. Of particular importance is an understanding of the interconnections between different targets within the same cancer, and observations of synergy between two different targeted therapies or between a targeted drug and conventional chemotherapy. It has become clear that many cancers are able to circumvent a single specific blockade, and pediatric leukemias are no exception in this regard. This review highlights the most promising approaches to new drugs and drug combinations for high-risk pediatric leukemia. Key biological evidence supporting selection of molecular targets is presented, together with a critical survey of recent progress toward the discovery, pre-clinical development, and clinical study of novel molecular therapeutics.

Small interfering RNA library screen of human kinases and phosphatases identifies polo-like kinase 1 as a promising new target for the treatment of pediatric rhabdomyosarcomas

Rhabdomyosarcoma, consisting of alveolar (aRMS) and embryonal (eRMS) subtypes, is the most common type of sarcoma in children. Currently, there are no targeted drug therapies available for rhabdomyosarcoma. In searching for new molecular therapeutic targets, we carried out genome-wide small interfering RNA (siRNA) library screens targeting human phosphatases (n = 206) and kinases (n = 691) initially against an aRMS cell line, RH30. Sixteen phosphatases and 50 kinases were identified based on growth inhibition after 72 hours. Inhibiting polo-like kinase 1 (PLK1) had the most remarkable impact on growth inhibition (approximately 80%) and apoptosis on all three rhabdomyosarcoma cell lines tested, namely, RH30, CW9019 (aRMS), and RD (eRMS), whereas there was no effect on normal muscle cells. The loss of PLK1 expression and subsequent growth inhibition correlated with decreased p-CDC25C and Cyclin B1. Increased expression of WEE 1 was also noted. The induction of apoptosis after PLK1 silencing was confirmed by increased p-H2AX, propidium iodide uptake, and chromatin condensation, as well as caspase-3 and poly(ADP-ribose) polymerase cleavage. Pediatric Ewing's sarcoma (TC-32), neuroblastoma (IMR32 and KCNR), and glioblastoma (SF188) models were also highly sensitive to PLK1 inhibition. Finally, based on cDNA microarray analyses, PLK1 mRNA was overexpressed (>1.5 fold) in 10 of 10 rhabdomyosarcoma cell lines and in 47% and 51% of primary aRMS (17 of 36 samples) and eRMS (21 of 41 samples) tumors, respectively, compared with normal muscles. Similarly, pediatric Ewing's sarcoma, neuroblastoma, and osteosarcoma tumors expressed high PLK1. We conclude that PLK1 could be a promising therapeutic target for the treatment of a wide range of pediatric solid tumors including rhabdomyosarcoma.

Reversal of oncogene transformation and suppression of tumor growth by the novel IGF1R kinase inhibitor A-928605

Background

The insulin-like growth factor (IGF) axis is an important signaling pathway in the growth and survival of many cell and tissue types. This pathway has also been implicated in many aspects of cancer progression from tumorigenesis to metastasis. The multiple roles of IGF signaling in cancer suggest that inhibition of the pathway might yield clinically effective therapeutics.

Methods

We describe A-928605, a novel pyrazolo [3,4-d]pyrimidine small molecule inhibitor of the receptor tyrosine kinases (IGF1R and IR) responsible for IGF signal transduction. This compound was first tested for its activity and selectivity via conventional in vitro kinome profiling and cellular IGF1R autophosphorylation. Additionally, cellular selectivity and efficacy of A-928605 were analyzed in an IGF1R oncogene-addicted cell line by proliferation, signaling and microarray studies. Finally, in vivo efficacy of A-928605 was assessed in the oncogene-addicted cell line and in a neuroblastoma model as a single agent as well as in combination with clinically approved therapeutics targeting EGFR in models of pancreatic and non-small cell lung cancers.

Results

A-928605 is a selective IGF1R inhibitor that is able to abrogate activation of the pathway both in vitro and in vivo. This novel compound dosed as a single agent is able to produce significant growth inhibition of neuroblastoma xenografts in vivo. A-928605 is also able to provide additive effects when used in combination with clinically approved agents directed against EGFR in non-small cell lung and human pancreatic tumor models.

Conclusion

These results suggest that a selective IGF1R inhibitor such as A-928605 may provide a useful clinical therapeutic for IGF pathway affected tumors and warrants further investigation.

Vascular determinants of cancer stem cell dormancy--do age and coagulation system play a role?

The inability of tumour-initiating cancer stem cells (CSCs) to bring about a net increase in tumour mass could be described as a source of tumour dormancy. While CSCs may be intrinsically capable of driving malignant growth, to do so they require compatible surroundings of supportive cells, growth factors, adhesion molecules and energy sources (e.g. glucose and oxygen), all of which constitute what may be referred to as a 'permissive' CSC niche. However, in some circumstances, the configuration of these factors could be incompatible with CSC growth (a 'non-permissive' niche) and lead to their death or dormancy. CSCs and their niches may also differ between adult and paediatric cancers. In this regard the various facets of the tumour-vascular interface could serve as elements of the CSC niche. Indeed, transformed cells with an increased tumour-initiating capability may preferentially reside in specific zones adjacent to tumour blood vessels, or alternatively originate from poorly perfused and hypoxic areas, to which they have adapted. CSCs themselves may produce increased amounts of angiogenic factors, or rely for this on their progeny or activated host stromal cells. It is likely that 'vascular' properties of tumour-initiating cells and those of their niches may diversify and evolve with tumour progression. The emerging themes in this area include the role of vascular (and bone marrow) aging, vascular and metabolic comorbidities (e.g. atherosclerosis) and the effects of the coagulation system (both at the local and systemic levels), all of which could impact the functionality of CSCs and their niches and affect tumour growth, dormancy and formation of occult as well as overt metastases. In this article we will discuss some of the vascular properties of CSCs relevant to tumour dormancy and progression, including: (i) the role of CSCs in regulating tumour vascular supply, i.e the onset and maintenance of tumour angiogenesis; (ii) the consequences of changing vascular demand (vascular dependence) of CSC and their progeny; (iii) the interplay between CSCs and the vascular system during the process of metastasis, and especially (iv) the impact of the coagulation system on the properties of CSC and their niches. We will use the oncogene-driven expression of tissue factor (TF) in cancer cells as a paradigm in this regard, as TF represents a common denominator of several vascular processes that commonly occur in cancer, most notably coagulation and angiogenesis. In so doing we will explore the therapeutic implications of targeting TF and the coagulation system to modulate the dynamics of tumour growth and tumour dormancy.