Advances in preclinical therapeutics development using small animal imaging and molecular analyses: the gastrointestinal stromal tumors model

Regorafenib in gastrointestinal stromal tumors

Gastrointestinal stromal tumors (GIST) are the most common mesenchymal tumors of the GI tract and constitute less than 1% of all digestive tract tumors--the stomach is the most common site. Regorafenib is a multi-tyrosine kinase inhibitor with regulatory approvals granted for colorectal cancers and GIST. The US FDA granted approval for the use of regorafenib in February 2013 in patients with advanced GIST for those who had failed on imatinib and sunitinib. This was based on a pivotal Phase III double-blind placebo controlled randomized trial that showed that there was a significant improvement in progression-free survival for patients on regorafenib.

Evaluating new therapies in gastrointestinal stromal tumor using in vivo molecular optical imaging

Gastrointestinal stromal tumors (GISTs) are the most common mesenchymal tumors in the US. The majority (~85%) of GISTs possess gain-of-function mutations in KIT or PDGFRA, causing constitutive activation of the kinase receptor. GIST management has been transformed by the identification of tumor driver mutations leading to unprecedented disease control of advanced GIST with the introduction of imatinib mesylate (IM). Despite IM's efficacy, most patients experience primary and/or secondary resistance within 2 y of treatment. Additional therapies and methods to optimize screening of novel approaches in preclinical studies are warranted. Clinically, treatment efficacy is typically assessed using Response Evaluation Criteria In Solid Tumors (RECIST) guidelines or Choi criteria. Both require a period of time on therapy before changes indicative of response can be observed. In addition, neither informs directly about cell death. We evaluated the use of molecular imaging technology in an animal model using near-infrared (NIR) imaging probes together with three-dimensional fluorescence molecular tomography (FMT) for assessing therapeutic response and ultimately optimizing our understanding of the biologic effects of these agents. We determined the potential of NIR probes (PSVue(TM) 794 and cell-penetrating KcapQ647) for detecting distinct markers of apoptosis and compare this to tumor size measured by MRI in response to IM treatment in GIST-T1 xenografts. Our studies revealed statistically significant increases in apoptosis due to IM treatment using both probes as early as 24 h post IM treatment which was confirmed by IHC. Molecular imaging will allow for faster and more effective screening of novel therapies in preclinical GIST models.

High CD133 expression levels in gastrointestinal stromal tumors

BACKGROUND

Gastrointestinal stromal tumours (GISTs) have activating KIT or PDGFRA gene mutations. Imatinib mesylate, which targets KIT and PDGFRA, is effective in treating GISTs, but 90% of GIST patients become imatinib-resistant as a result of acquiring secondary KIT mutations. Recent findings suggest that tumour growth can be driven by mutated self-renewing progenitors known as cancer stem cells (CSCs), which are believed to be present in all neoplastic proliferations and are thought to accumulate mutations. It is therefore possible that the acquisition of secondary KIT mutations during imatinib treatment may occur in putative GIST CSCs.

METHODS

Using flow cytometry, in vivo murine xenografts and molecular characterization, we tried to identify putative GIST CSCs by looking for the occurrence of common CSC markers such as KIT, CD133, CD90, CD44, and CD34 in 18 surgical samples obtained from nine untreated and nine imatinib-treated KIT-mutated GIST patients.

RESULTS

The results indicated the homogeneous and previously unreported expression of CD133 (18/18), CD90 (15/16), and CD44 (12/14), together with KIT (18/18) and CD34 (13/18). This profile is similar to that identified in bone marrow mesenchymal progenitors and does not seem to be significantly modified by imatinib as only marginal changes in KIT and CD133 expression (P ≤ 0.05, Mann-Whitney test) were found in the treated samples.

CONCLUSIONS

These findings suggest that GISTs are a clonal expansion of quite primitive cells that strictly depend on KIT oncogenic addiction, and have no cancer/stem cell component that can be detected by means of the antigens used in this study.

Preclinical evaluation of KIT/PDGFRA and mTOR inhibitors in gastrointestinal stromal tumors using small animal FDG PET

Background

Primary and secondary drug resistance to imatinib and sunitinib in patients with gastrointestinal stromal tumors (GISTs) has led to a pressing need for new therapeutic strategies such as drug combinations. Most GISTs are caused by mutations in the KIT receptor, leading to upregulated KIT tyrosine kinase activity. Imatinib and nilotinib directly inhibit the kinase activity of KIT, while RAD001 (everolimus) inhibits mTOR. We report a preclinical study on drug combinations in a xenograft model of GIST in which effects on tumor dimensions and metabolic activity were assessed by small animal PET imaging.

Methods

Rag2-/-; γcommon -/- male mice were injected s.c. into the right leg with GIST 882. The animals were randomized into 6 groups of 6 animals each for different treatment regimens: No therapy (control), imatinib (150 mg/kg b.i.d.) by oral gavage for 6 days, then once/day for another 7 days, everolimus (10 mg/kg/d.) by oral gavage, everolimus (10 mg/kg/d.) + imatinib (150 mg/kg b.i.d.) by oral gavage for 6 days, then once/day for another 7 days, nilotinib (75 mg/kg/d.) by oral gavage, nilotinib (75 mg/kg/d.) + imatinib (150 mg/kg b.i.d) by oral gavage for 6 days, then once/day for another 7 days. Tumor growth control was evaluated by measuring tumor volume (cm³). Small animal PET (GE Explore tomography) was used to evaluate tumor metabolism and performed in one animal per group at base-line then after 4 and 13 days of treatment.

Results

After a median latency time of 31 days, tumors grew in all animals (volume 0,06-0,15 cm³) and the treatments began at day 38 after cell injection. Tumor volume control (cm3) after 13 days of treatment was > 0.5 for imatinib alone and nilotinib alone, and < 0.5 for the 2 combinations of drugs and for everolimus alone. The baseline FDG uptake was positive in all animals. FDG/SUV/TBR was strongly reduced over time by everolimus both as a single agent and in combination with imatinib respectively: 3.1 vs. 2.3 vs. 1.9 and 2.5 vs 2.3 vs 0.

Conclusions

As single agents, all drugs showed an anti-tumor effect in GIST xenografts but everolimus was superior. The everolimus plus imatinib combination appeared to be the most active regimen both in terms of inhibiting tumor growth and tumor metabolism. The integration of everolimus in GIST treatment merits further investigation.

The emerging role of insulin-like growth factor 1 receptor (IGF1r) in gastrointestinal stromal tumors (GISTs)

Recent years have seen a growing interest in insulin-like growth factor 1 receptor (IGF1R) in medical oncology. Interesting data have been reported also on IGF1r in gastrointestinal stromal tumors (GISTs) especially in children and in young adult patients whose disease does not harbour mutations on KIT and PDGFRA and are poorly responsive to conventional therapies. However, it is too early to reach conclusions on IGF1R as a novel therapeutic target in GIST because the receptor's biological role is still to be defined and the clinical significance in patients needs to be studied in larger studies. We update and comment the current literature on IGF1R in GISTs and discuss the future perspectives in this promising field.

Combined treatment strategies in gastrointestinal stromal tumors (GISTs) after imatinib and sunitinib therapy

Resistance to tyrosine-kinase inhibitors remains an open issue in the treatment of patients with gastrointestinal stromal tumors. The complex biology of disease in the multi-resistant setting has led a progressively growing urgency and interest in development combined or integrated therapies. This mini-review outlines the rationale for developing new combined therapeutic approaches, and describes the state of the art of the various potential strategies and the promising research perspectives.