Abstract CN02-02: Dual inhibition of mTOR and IGFR pathways.
Authors: Ebbinghaus Scot W. 
Published: June 2012

There has been intense interest by academic investigators and pharmaceutical companies in the development of investigational combinations to simultaneously inhibit both insulin-like growth factor receptor 1 (IGF-1R) and the mammalian target of rapamycin (mTOR). The development of one of these combinations, the rapamycin analog ridaforolimus and the anti-IGF-1R antibody dalotuzumab, provides an illustrative case study in the rational selection and development of a targeted therapy combination for the treatment of cancer. This review will discuss success factors for the selection of a combination for clinical development and the identification of a potentially responsive patient population. The ideal combinations for clinical development will satisfy three important criteria: 1) compelling scientific pathway rationale for the combination; 2) independent identification of the combination from unbiased high throughput screening; and 3) confirmation of combination benefit based on preclinical additive or synergistic activity in vitro and in vivo. The intersection of these three criteria together with a reasonable expectation of clinical feasibility based on available data will provide the best combinations to bring forward to clinical development.

Dual inhibition of IGF-1R and mTOR is based on the compelling pathway rationale. Vertical inhibition of multiple nodes in the phosphotidyl inositol-3-kinase (PI3K) pathway blocks compensatory feedback activation and potentiates PI3K pathway inhibition. IGF-1R is a receptor tyrosine kinase that sits upstream of mTOR in the PI3K pathway, and in principle, hitting two nodes in this pathway could provide stronger inhibition of the pathway and render more tumors susceptible. Moreover, rapamycin analogs inhibit the TORC1 complex, leaving TORC2 unopposed and leading to a now well-described feedback activation of the AKT oncogene via signaling through the IGF-1R and the insulin receptor substrate (IRS). While the clinical consequences of AKT activation as a result of mTOR inhibition are as yet unproven, it has been shown that AKT phosphorylation is a clinically observable phenomenon following treatment with the mTOR inhibitor everolimus in women with breast cancer. The activation of AKT following rapamycin analog treatment could represent a mechanism of resistance to mTOR inhibitors. The beneficial effects of combining ridaforolimus and dalotuzumab on the PI3K pathway was reported, demonstrating that while ridaforolimus lead to a marked increase in pAKT in vitro, the combination of ridaforolimus and dalotuzumab reduced pAKT levels to baseline levels, and resulted in enhanced suppression of downstream PI3K targets such as pPRAS40, FOXO3A and S6 ribosomal protein. Collectively with data in the literature, these studies provided compelling scientific pathway rationale for this combination. Independently, RNA interference enhancer screens at Merck predating the development of the pathway rationale had identified the combination of IGF-1R and mTOR inhibitors as one of high interest. A short hairpin (sh)RNA kinome library screen identified FRAP1 (mTOR) as a top hit to enhance the suppression of proliferation in HT-29 cells by dalotuzumab. Conversely, a short interfering (si)RNA genomic screen using HT-1080 cells treated with ridaforolimus identified IGF-1R and several components of the IGF-1R pathway as the primary mediator of the AKT feedback activation noted with ridaforolimus treatment. Together, these unbiased enhancer screen results underscore the centrality of the IGF1R-mTOR relationship in tumor proliferation and feedback regulation of AKT.

In pre-clinical studies, the combination of ridaforolimus and dalotuzumab showed additive or synergistic growth inhibition in many, but not all cell lines and tumor xenograft models tested. Importantly, these preclinical studies confirmed that the presence of IGF-1R and the activation of the IGF-1R pathway were necessary for combination benefit.

The ridaforolimus-dalotuzumab combination did not undergo formal preclinical toxicology studies, but clinical feasibility was considered highly probable. Both single agents were members of a well-studied class and had been through Phase 1 studies with ongoing Phase 2–3 studies and a well-characterized safety profile. The agents did not have overlapping toxicities except for hyperglycemia which is readily monitored and treated in the clinical setting. Because one agent was a small molecule targeting an intracellular protein while the other was a monoclonal antibody targeting a cell surface receptor, pharmacological drug-drug interactions were considered unlikely, although the possibility of pharmacodynamic interaction enhancing on target toxicities of either agent was acknowledged. Finally, the practical consideration that the humanized monoclonal antibody did not bind to IGF-1R in species below primates made non-clinical toxicology studies impractical. Thus, the ridaforolimus-dalotuzumab combination had compelling biological rationale based on knowledge of the pathway, had been selected as top hits based on unbiased enhancer screens, had demonstrated pre-clinical additivity or synergy in several tumor models, and was predicted to be clinically feasible.

The Phase 1 study of the ridaforolimus-dalotuzumab combination was reported, and showed that the combination was tolerable. The most common and dose limiting toxicity of the combination was stomatitis, similar to that of the single agent mTOR inhibitor. The recommended dose was nearly that of the full doses of both single agents, but required a slight (25%) reduction of the ridaforolimus from full doses as a single agent, suggesting that some adverse events may be potentiated by the addition of the dalotuzumab. Hyperglycemia was common, but manageable with close monitoring and proactive management with glucose lowering agents. Other common clinical adverse events included asthenia, anorexia, diarrhea, and skin rash, mostly mild to moderate in severity. Common laboratory based adverse events included thrombocytopenia and hyperlipidemias, also generally mild to moderate in severity. Phase 1 studies with other mTOR inhibitor and anti-IGF-1R combinations have generally shown similar tolerability and adverse event profiles.

Preliminary evidence of anti-tumor activity was observed for the ridaforolimus-dalotuzumab combination. Objective responses were reported in patients with breast cancer, non-small cell lung cancer, ovarian cancer, and other solid tumors. Among women with breast cancer, the activity signal appeared to derive predominantly from patients with estrogen receptor (ER) positive and high proliferation (based on assessment of Ki67 from the primary tumor) luminal B-like tumors that were refractory to endocrine therapy. Among these patients, the objective response rate was 27% (3 of 11 patients), but other evidence of anti-tumor activity included declines in tumor markers (CA15.3), metabolic activity by FDG-PET scanning, and prolonged stable disease for greater than 6 months. Collectively, 6 of 11 patients with ER+ high proliferation breast cancer had objective evidence of the activity of the combination based on one or more of these parameters. In other mTOR inhibitor plus anti-IGF-1R combination studies, objective evidence of anti-tumor activity has been reported for patients with Ewing sarcoma and certain other sarcoma subtypes and potentially in other solid tumors.

A Phase 2 study with the ridaforolimus-dalotuzumab combination is currently ongoing in patients with ER+ high proliferation breast cancer progressing on aromatase inhibitor therapy. Current efforts are directed at rationalizing the activity signal in luminal B-like breast cancer and identification of other tumor types that may respond to dual inhibition of IGF-1R and mTOR pathways. Several features of luminal B breast cancer may explain the responsiveness of this tumor type to the ridaforolimus-dalotuzumab combination. First, IGF-1R and molecules associated with IGF-1R signaling are overexpressed in a subset of ER+ positive breast cancers. IGF-1R overexpression has been reported in association with tamoxifen resistance in pre-clinical models, and data from clinical breast cancer specimens suggest that both the IGF-1 ligand and IGF-1R tend to be highly expressed in ER+ high proliferation or luminal B breast cancers. Preclinical studies suggest that RAS pathway activity correlates with resistance to ridaforolimus, and among breast cancer subtypes, RAS pathway activity is highest in triple negative disease, a subtype that is relatively resistant to ridaforolimus in pre-clinical studies. In contrast, ER+ high proliferation breast cancer is generally low in RAS pathway activity and tends to be more responsive to ridaforolimus. These observations lead to a hypothesis that ER+ high proliferation breast cancer is characterized by features indicative of high PI3K pathway activity, relatively low utilization of ER leading to endocrine resistance, high expression of IGF-1R family members, and low RAS pathway activity rendering this tumor type susceptible to dual inhibition of IGF-1R and mTOR pathways with the ridaforolimus-dalotuzumab combination. These hypotheses and clinical observations of activity in ER+ high proliferation breast cancer suggest a potential method for identification of indications for that may respond to a novel targeted combination therapy by identifying the overlap of responder markers for each single agent.

In summary, the ridaforolimus-dalotuzumab combination represents an interesting case study in oncology combination therapy development, and illustrates the key principles in the selection of a clinically viable combination and the identification of potentially responsive tumor types.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2011 Nov 12-16; San Francisco, CA. Philadelphia (PA): AACR; Mol Cancer Ther 2011;10(11 Suppl):Abstract nr CN02-02.