Abstract 1613: In vitro acquired tumor cell resistance to sunitinib contributes to an in vivo resistant phenotype.
Authors: Gotink Kristy J. , Pili Roberto , Honeywell Richard J. , Griffioen Arjan W. , Peters Godefridus J. , Broxterman Henk J. , Verheul Henk MW 
Published: August 2014

Abstract
Background: Resistance to sunitinib and other TKIs is a major clinical problem. Previously we have shown that tumor cells acquire transient resistance in vitro by continuous incubation with sunitinib and found that in resistant tumor cells sunitinib is sequestered in intracellular lysosomes as the potential underlying mechanism of resistance. In vivo resistance to sunitinib may be related to its anti-angiogenic action and/or direct tumor cell-directed action. Here we studied the mechanism of resistance to sunitinib in vivo by comparing in vitro established sunitinib resistant HT-29 tumor cells (HT-29 SUN) with their parental cells (HT-29 PAR).
Methods: From HT-29 SUN and HT-29 PAR cell lines, tumors were established in severe combined immunodeficient (SCID) mice by subcutaneous injection or by tumor transplantation from sacrificed mice into a new cohort of mice with n=6 and n=8 mice per treatment group respectively. Mice were treated with sunitinib malate, provided by Pfizer Inc., at a dose of 40 mg/kg/d or vehicle. Tumor growth was monitored in time and tumor weights were measured after sacrificing the mice. Sunitinib concentrations were measured by LC-MS/MS. In addition, microvessel density (MVD) and proliferation rate of tumor cells were determined by immunohistochemical staining of CD31 and ki67.
Results: Tumor growth inhibition by sunitinib of HT-29 SUN and HT-29 PAR tumor bearing mice revealed comparable resistance of HT-29 SUN in vivo (23-38% growth inhibition; p=ns) to in vitro, while sunitinib significantly inhibited growth of HT-29 PAR (74-79%; p&lt;0.01) in two independent experiments. The intratumoral concentration of sunitinib in HT-29 SUN and HT-29 PAR treated tumors was comparable (8.5 ± 2.8 vs 9.2 ± 1.9 μM (mean ± SD; n=8; p=ns). MVD was significantly reduced in both HT-29 SUN and HT-29 PAR sunitinib treated tumors compared to control (HT-29 SUN: 3.8 ± 2.6 vs 9.3 ± 4.8 vessels for sunitinib vs vehicle treatment (p&lt;0.05), HT-29 PAR: 4.7 ± 1.7 vs 10.9 ± 4.5 vessels per field, respectively (p&lt;0.01)). In contrast, ki67 staining revealed no inhibition of tumor cell proliferation in HT-29 SUN treated tumors (57 ± 17 vs 61 ± 14 % for sunitinib vs vehicle treatment (p=ns)), while in HT-29 PAR tumors proliferation was significantly reduced upon sunitinib treatment (52 ± 8 vs 88 ± 5 %, respectively (p&lt;0.001)).
Conclusions: Our findings suggest that sunitinib resistance in this model is tumor cell- rather than host-mediated and is independent of intratumoral drug delivery. Further analyses of potential lysosomal involvement in this in vivo resistance mechanism are ongoing.
Citation Format: Kristy J. Gotink, Roberto Pili, Richard J. Honeywell, Arjan W. Griffioen, Godefridus J. Peters, Henk J. Broxterman, Henk MW Verheul. In vitro acquired tumor cell resistance to sunitinib contributes to an in vivo resistant phenotype. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 1613. doi:10.1158/1538-7445.AM2013-1613