The Enhanced In Vivo Activity of the Combination of a MEK and a PI3K Inhibitor Correlates with [18F]-FLT PET in Human Colorectal Cancer Xenograft Tumour-Bearing Mice

Abstract

Combined targeting of the MAPK and PI3K signalling pathways in cancer may be necessary for optimal therapeutic activity. To support clinical studies of combination therapy, 3′-deoxy-3′-[18F]-fluorothymidine ([18F]-FLT) uptake measured by Positron Emission Tomography (PET) was evaluated as a non-invasive surrogate response biomarker in pre-clinical models. The in vivo anti-tumour efficacy and PK-PD properties of the MEK inhibitor PD 0325901 and the PI3K inhibitor GDC-0941, alone and in combination, were evaluated in HCT116 and HT29 human colorectal cancer xenograft tumour-bearing mice, and [18F]-FLT PET investigated in mice bearing HCT116 xenografts. Dual targeting of PI3K and MEK induced marked tumour growth inhibition in vivo, and enhanced anti-tumour activity was predicted by [18F]-FLT PET scanning after 2 days of treatment. Pharmacodynamic analyses using the combination of the PI3K inhibitor GDC-0941 and the MEK inhibitor PD 0325901 revealed that increased efficacy is associated with an enhanced inhibition of the phosphorylation of ERK1/2, S6 and 4EBP1, compared to that observed with either single agent, and maintained inhibition of AKT phosphorylation. Pharmacokinetic studies indicated that there was no marked PK interaction between the two drugs. Together these results indicate that the combination of PI3K and MEK inhibitors can result in significant efficacy, and demonstrate for the first time that [18F]-FLT PET can be correlated to the improved efficacy of combined PI3K and MEK inhibitor treatment.

Enhanced anti-tumour activity of the combination of the novel MEK inhibitor WX-554 and the novel PI3K inhibitor WX-037

“…This improved activity is consistent with previous studies using combinations of PI3K and MEK inhibitors in a range of pre-clinical human tumour xenograft and mouse models [19, 21, 22, 25, 55–58]. In a previous study using the MEK inhibitor, PD 0325901, and the PI3K inhibitor, GDC-0941, the cell line-dependent differences in sensitivity to the inhibitors determined in the in vitro studies did not correlate with the results observed in the in vivo studies [23, 25], suggesting that HT29 xenografts would not necessarily of been more sensitive to the inhibitors in vivo. Nevertheless, previous studies by UCB and Wilex have demonstrated that WX-554 treatment is able to induce tumour growth delay in HT29 xenografts [5].…”

“…Furthermore, single agent WX-554 induced tumour growth delay at 2 mg/kg in HCT116 colorectal tumour xenograft models, consistent with previous unpublished studies using WX-554 and published studies using other MEK inhibitors that reported tumour growth delay or stasis in vivo, with increased sensitivity in BRAF or RAS mutant cells and tumours [3–5, 25]. In vivo pharmacokinetic analyses revealed concentrations of WX-554 in plasma and tumour tissue increased in a dose-dependent manner and that tumour WX-554 levels generally exceeded the in vitro GI 50 concentration, whereas plasma levels were more variable.…”

“…Pharmacokinetic studies indicated that concentrations of WX-037 in plasma and tumour tissue increased dose dependently and that WX-037 tumour and plasma levels generally exceeded the in vitro GI 50 concentration at an early time point (6 h), depending on the cell line and dose, whereas levels generally had decreased below the GI 50 value by the later time point (24 h). These pharmacokinetic data are similar to those observed in HCT116 tumour xenograft-bearing mice treated with pictilisib [25]. …”

“…However, concomitant dosing of WX-037 with the high dose (5 mg/kg) of WX-554 caused a dose-dependent decrease in early time point (6 h) WX-554 tumour and plasma concentrations, and a dose-dependent increase in late time point (24 h) WX-554 levels, suggesting that the presence of WX-037 may delay the uptake of WX-554, leading to higher WX-554 levels at 24 h. Furthermore, concomitant WX-554 dosing with high doses (100 mg/kg) of WX-037 caused a dose-dependent decrease in early time point (6 h) WX-037 tumour and plasma concentrations; however, this effect was only significant in the HCT116 tumour tissue due to variation in the data. The pharmacokinetic interaction between higher doses of WX-037 and WX-554 is in contrast to the results obtained with the combination of PD 0325901 and pictilisib in HCT116 tumour xenograft-bearing mice [25], where there was no significant pharmacokinetic interaction, and to pre-clinical and clinical reports that there was no pharmacokinetic interaction between the PI3K and MEK inhibitors, pictilisib and cobimetinib [52, 53]. However, a recent Phase Ib study using a combination of the PI3K inhibitor buparlisib and the MEK inhibitor trametinib reported a potential pharmacokinetic interaction, which was suggested to be due to the variability of the accumulation of the MEK inhibitor as no clear mechanism could be identified [54].…”

Pharmacodynamic Biomarker Development for PI3K Pathway Therapeutics

“…18 F-FLT PET is used for detecting antiproliferative effects, since it is a thymidine analog, whose accumulation in cells is determined by the expression and activity of the enzyme thymidine kinase 1 and specific nucleoside transporters, both of which are under the control of S-phase cell cycle regulators. 98 Furthermore, the uptake of 18 F-FLT PET has been shown to correlate with standard proliferation markers, such as Ki67, TK1, and BrdU uptake. [99][100][101][102] Using 18 F-FLT PET, changes in proliferation compared to baseline have been demonstrated in a variety of human tumor xenografts as early as 18, 24, and 120 hours after using either single-agent class I selective PI3K inhibitor GDC-0941 (pictilisib) or MEK inhibitor PD0325901.…”