It is possible that toward the end of the experiment the antiangiogenic effects of enzastaurin on the CAL27 xenografted tumors were diminishing, and thus no difference in MVD was demonstrated compared with control. In fact, the slope of the tumor growth curves between control and enzastaurintreated mice appears to be similar over HA-1077 the last 4 days of the experiment, suggesting that the CAL27 tumors were becoming resistant to enzastaurin. Moreover, despite observing a significant increase in apoptosis when rapamycin and enzastaurin were combined in CAL27 cells in vitro, apoptosis induction in vivo appeared to be mostly mediated by rapamycin.
Nonetheless, within all the parameters we surveyed in vivo, the combination of both agents yielded the greatest desired effect, and notably Cinacalcet clinical trial tumors in mice treated with enzastaurin and rapamycin continued to decrease in size throughout the experiment. This suggests that relatively small but complementary effects on multiple processes when rapamycin and enzastaurin are combined result in the greatest antitumor efficacy.Because synergy was demonstrated in vitro, one might have expected a greater effect of the combination in vivo. Discrepancies between in vitro and in vivo results are often observed and can be difficult to predict. Stromal tumor interactions, as well as growth in three dimensions and within a biologic platform do not exist in vitro and could all account for the differential effects. It appears that, although both agents exert inhibition of relevant and complementary pathways in vitro, it becomes less important in the xenograft model.
The effects of each agent on angiogenesis underscore this hypothesis, demonstrating the importance of inhibiting stromal cells in addition to tumor cells. In addition, reactive stromal cells themselves can secrete paracrine growth factors that stimulate tumor growth and lead Cinacalcet structure to therapeutic resistance perhaps by bypassing the putative targets of either agent. More sophisticated 3 dimensional in vitro models incorporating stromal elements may help to elucidate these possibilities but, ultimately, the proof must come in vivo and eventually in patients. These data also raise several unanswered questions and potential future directions. The in vivo experiments were performed in a single cell line known to harbor some sensitivity to rapamycin.
It would be valuable to conduct tumor growth experiments in models with known resistance to either rapamycin or enzastaurin to determine whether antitumor efficacy would be observed. Furthermore, the current data demonstrate effects on angiogenesis, proliferation, and apoptosis; however, alternative mechanisms Cinacalcet solubility of cell death, such as autophagy, may also health insurance be relevant, especially considering the role of mTOR as a negative regulator of autophagy.15 Takeuchi et al16 have demonstrated in malignant glioma cells resistant to rapamycin that autophagy is augmented by the addition of a phosphoinositide 3 kinase inhibitor or UCN 01. The latter agent is similar but less selective than enzastaurin, suggesting that parallel mechanisms are operational when rapamycin and enzastaurin are combined. This study therefore supports further development of enzastaurin and rapamycin in SCCHN.