An evaluation of 2-deoxy-2-[18F]fluoro-D-glucose and 3'-deoxy-3'-[18F]-fluorothymidine uptake in human tumor xenograft models.

PURPOSE: The aim of this study is to assess the variability of 2-deoxy-2-[(18)F]fluoro-D: -glucose ([(18)F]-FDG) and 3'-deoxy-3'-[(18)F]-fluorothymidine ([(18)F]-FLT) uptake in pre-clinical tumor models and examine the relationship between imaging data and related histological biomarkers. PROCEDURES: [(18)F]-FDG and [(18)F]-FLT studies were carried out in nine human tumor xenograft models in mice. A selection of the models underwent histological analysis for endpoints relevant to radiotracer uptake. Comparisons were made between in vitro uptake, in vivo imaging, and ex vivo histopathology data using quantitative and semi-quantitative analysis. RESULTS: In vitro data revealed that [1-(14)C]-2-deoxy-D: -glucose ([(14)C]-2DG) uptake in the tumor cell lines was variable. In vivo, [(18)F]-FDG and [(18)F]-FLT uptake was highly variable across tumor types and uptake of one tracer was not predictive for the other. [(14)C]-2DG uptake in vitro did not predict for [(18)F]-FDG uptake in vivo. [(18)F]-FDG SUV was inversely proportional to Ki67 and necrosis levels and positively correlated with HKI. [(18)F]-FLT uptake positively correlated with Ki67 and TK1. CONCLUSION: When evaluating imaging biomarkers in response to therapy, the choice of tumor model should take into account in vivo baseline radiotracer uptake, which can vary significantly between models.

Blocking phosphoinositide 3-kinase activity in colorectal cancer cells reduces proliferation but does not increase apoptosis alone or in combination with cytotoxic drugs.

In response to growth factors, class IA phosphoinositide 3-kinases (PI3K) phosphorylate phosphatidylinositol-4,5-bisphosphate, converting it to phosphatidylinositol-3,4,5-trisphosphate to activate protein kinase B/Akt. This is widely reported to promote tumorigenesis via increased cell survival, proliferation, migration, and invasion, and many tumor types, including colorectal cancer, exhibit increased PI3K signaling. To investigate the effect of inhibiting PI3K and as an alternative to the use of small molecular inhibitors of PI3K with varying degrees of selectivity, HT29 and HCT116 colorectal cancer cells bearing mutant PIK3CA were generated that could be induced with doxycycline to express synchronously a dominant negative subunit of PI3K, Deltap85alpha. On induction, decreased levels of phosphorylated protein kinase B were detected, confirming PI3K signaling impairment. Induction of Deltap85alpha in vitro reduced cell number via accumulation in G(0)-G(1) phase of the cell cycle in the absence of increased apoptosis. These effects were recapitulated in vivo. HT29 cells expressing Deltap85alpha and grown as tumor xenografts had a significantly slower growth rate on administration of doxycycline with reduced Ki67 staining without increased levels of apoptotic tissue biomarkers. Furthermore, in vitro Deltap85alpha expression did not sensitize HT29 cells to oxaliplatin- or etoposide-induced apoptosis, irrespective of drug treatment schedule. Further analysis comparing isogenic HCT116 cells with and without mutation in PIK3CA showed no effect of the mutation in either proliferative or apoptotic response to PI3K inhibition. These data show in colorectal cancer cells that PI3K inhibition does not provoke apoptosis per se nor enhance oxaliplatin- or etoposide-induced cell death.

Neuroendocrine stress but not feeding responses to centrally administered neuropeptide Y are suppressed in pregnant rats.

Metabolic peptides such as orexin and neuropeptide Y (NPY) exert profound effects on feeding but also act centrally to stimulate the hypothalamo-pituitary-adrenal (HPA) axis. In late pregnancy the HPA axis is hyporesponsive to centrally administered orexin-A, which signals to the HPA axis, in part, via arcuate NPY neurones. We investigated whether reduced HPA axis responses to orexin may be a consequence of down-regulated NPY signaling to the paraventricular nucleus (PVN) in pregnancy. Pregnant (d 21) and virgin rats were blood sampled for ACTH, corticosterone, and oxytocin (also a stress hormone in rats) before and after intracerebroventricular NPY or vehicle. Behavior was monitored. Rats were killed 4 h after NPY and brains removed for in situ hybridization. In another experiment rats were given vehicle or NPY, perfuse fixed 90 min later, and brain sections processed for Fos and oxytocin immunocytochemistry. NPY significantly increased ACTH, corticosterone and oxytocin secretion in the virgins but had no such effect on ACTH or oxytocin in the pregnant rats; the corticosterone response to NPY was markedly attenuated in pregnant rats. NPY increased CRH and vasopressin mRNA expression in the parvocellular PVN and stimulated Fos expression in magnocellular supraoptic and PVN oxytocin neurones of virgin but not pregnant rats. NPY increased food intake and drinking similarly in virgin and pregnant rats. Thus, neuroendocrine stress responses to central NPY are absent in late pregnancy, whereas ingestive behavioral responses are intact. These changes may explain the similarly attenuated HPA response to centrally administered orexin-A and will favor anabolic adaptations in pregnancy.