Investigating the role of Class Ia phosphoinositide-3 kinase isoforms in Mantle Cell Lymphoma.
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Mantle Cell Lymphoma (MCL) is a rare but aggressive Non-Hodgkin Lymphoma (NHL). Although t(11;14) is a hallmark of MCL, it is insufficient for lymphomagenesis. The phosphoinositide-3 kinase (PI3K) pathway is thought to play an important role in MCL pathogenesis and the PI3K p110δ isoform is enriched in leucocytes making it an attractive target. Early phase trials evaluating the p110δ selective inhibitor GS-1101 however demonstrate inferior responses in MCL compared to chronic lymphocytic leukaemia and indolent NHL. The relative contribution of the class Ia PI3K isoforms p110α (PIK3CA), p110β (PIK3CB) and p110δ (PIK3CD) was therefore evaluated in MCL. Immunohistochemistry on MCL tissue microarrays revealed that while p110δ was highly expressed, p110α showed wide variation and p110β expression was the weakest. A significant increase in p110α expression was found with disease progression. Although GS-1101 was sufficient to abolish B-cell receptor mediated PI3K activation, additional p110α inhibition was necessary to abolish constitutive PI3K activation in MCL exhibiting high p110α expression. Compared to GS-1101, GDC-0941 (p110α and p110δ inhibitor) had greater in vitro toxicity and a high PIK3CA/PIK3CD mRNA ratio (> twice ratio in healthy B-cells) was able to identify primary MCL samples that were resistant to GS-1101 but significantly more sensitive to GDC—0941. This ratio also increased with disease progression. No PIK3CA or PIK3R1 activating mutations were found. In summary, blockade of both p110α and p110δ appears to be necessary for effective PI3K inhibition in MCL, particularly with relapse. The PIK3CA/PIK3CD mRNA ratio may help identify those patients that are most likely to respond. Finally, a disseminated xenograft model of human primary MCL was established in NSG mice. Engraftment of primary MCL was demonstrated by peripheral blood flow cytometry, tissue immunohistochemistry and FISH for t(11;14). This model is potentially valuable for pre-clinical in vivo testing of novel drugs for this incurable disease.
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