Clinical activity of mammalian target of rapamycin inhibitors in solid tumors

The phosphatidylinositol 3-kinase (PI3K)-Akt-mammalian target of rapamycin (mTOR) pathway is vital for cell metabolism, growth, and proliferation. mTOR is frequently upregulated in many tumor types and hence has become an important target in cancer treatment. Sirolimus and its derivatives (rapalogs) interact with the intracellular receptor FK506 binding protein 12 (FKBP12), forming a complex with high affinity for mTOR and thus disrupting its activity. Rapalogs are being evaluated extensively in cancer patients with different formulations and schedules. Significant clinical activity has led to their approval for the treatment of kidney cancer, mantle cell lymphoma, and subependymal giant cell astrocytoma; however, despite increasing knowledge about cancer cell biology, their activity in other malignancies is unclear. Further research is needed to identify optimal dosage, administration and targeted combination as well as the subset of patients likely to respond to mTOR/PI3K inhibition. This review focuses on a discussion of the pathway, its implications in cancer biology and results of clinical trials of rapalogs alone or in combination, organizing them by common malignancy type.

Temsirolimus has activity in non-mantle cell non-Hodgkin's lymphoma subtypes: The University of Chicago phase II consortium

PURPOSE

Despite high initial remission rates, most lymphomas relapse and require further therapy. The mammalian target of rapamycin (mTOR) pathway is a validated target in mantle cell lymphoma, but has not been extensively evaluated in other lymphomas.

PATIENTS AND METHODS

We performed a phase II trial of single-agent temsirolimus 25-mg weekly in patients with relapsed aggressive and indolent lymphomas. The primary objective was overall and complete response rate. Patients were stratified by histology: group A (diffuse large B-cell lymphoma, transformed follicular lymphoma), group B (follicular lymphoma), and group C (chronic lymphocytic leukemia/small lymphocytic lymphoma, and other indolent lymphomas).

RESULTS

Eighty-nine patients were treated, with outcome strongly dependent on histology. Group A had an overall and complete response rate of 28.1% and 12.5%, respectively, and median progression-free survival (PFS) of 2.6 months and median overall survival (OS) of 7.2 months. Group B had overall and complete response rates of 53.8% and 25.6%, respectively, and median PFS of 12.7 months; median OS has not yet been reached. Group C had a partial response rate of 11% with no complete responders. Toxicity was mainly mild and/or reversible myelosuppression and mucositis; however, four patients developed pneumonitis.

CONCLUSIONS

Single-agent temsirolimus has significant activity in both diffuse large B-cell lymphoma and follicular lymphoma, although the durability of responses and PFS are longer for patients with follicular lymphoma. This is the first report of substantial activity of temsirolimus in lymphomas other than mantle cell lymphoma, and supports further evaluation of mTOR as a target in these diseases.

mTOR signaling and drug development in cancer

Mammalian target of rapamycin (mTOR) is a protein kinase of the PI3K/Akt signaling pathway. Activation of mTOR in response to growth, nutrient and energy signals leads to an increase in protein synthesis, which is required for tumor development. This feature makes mTOR an attractive target for cancer therapy. First-generation mTOR inhibitors are sirolimus derivatives (rapalogs), which have been evaluated extensively in cancer patients. Everolimus and temsirolimus are already approved for the treatment of renal-cell carcinoma. Temsirolimus is also approved for the treatment of mantle-cell lymphoma. These drugs, in addition to ridaforolimus (formerly deforolimus) and sirolimus, are currently being evaluated in clinical trials of various cancers. Second-generation mTOR inhibitors are small molecules that target the kinase domain, and have also entered clinical development. Clinical trials are underway to identify additional malignancies that respond to mTOR inhibitors, either alone or in combination with other therapies. Future research should evaluate the optimal drug regimens, schedules, patient populations, and combination strategies for this novel class of agents.

The future of small molecule inhibitors in lymphoma

For the many patients with lymphoma that has relapsed after and/or has become refractory to existing treatments, the development of novel therapeutics is imperative. Investigation into intracellular processes that are dysregulated during lymphomagenesis has uncovered several new potential targets for anticancer agents. Although monoclonal antibodies and other immunotherapeutics have led to dramatic advances in the treatment of patients with lymphoma, the parallel development of small molecule inhibitors has been equally exciting. These agents, whose small size allows direct entry into tumor cells, can target distinct proteins or complexes, thereby disrupting molecular processes on which neoplastic cells depend for survival and growth. This review surveys the published literature on many of these new targeted molecules, focusing on some of the most promising agents for which phase 2 data currently exist. It also explores the potential for incorporating these agents into broader multidrug regimens.

Phase III study to evaluate temsirolimus compared with investigator's choice therapy for the treatment of relapsed or refractory mantle cell lymphoma

PURPOSE

Temsirolimus, a specific inhibitor of the mammalian target of rapamycin kinase, has shown clinical activity in mantle cell lymphoma (MCL). We evaluated two dose regimens of temsirolimus in comparison with investigator's choice single-agent therapy in relapsed or refractory disease.

PATIENTS AND METHODS

In this multicenter, open-label, phase III study, 162 patients with relapsed or refractory MCL were randomly assigned (1:1:1) to receive one of two temsirolimus regimens: 175 mg weekly for 3 weeks followed by either 75 mg (175/75-mg) or 25 mg (175/25-mg) weekly, or investigator's choice therapy from prospectively approved options. The primary end point was progression-free survival (PFS) by independent assessment.

RESULTS

Median PFS was 4.8, 3.4, and 1.9 months for the temsirolimus 175/75-mg, 175/25-mg, and investigator's choice groups, respectively. Patients treated with temsirolimus 175/75-mg had significantly longer PFS than those treated with investigator's choice therapy (P = .0009; hazard ratio = 0.44); those treated with temsirolimus 175/25-mg showed a trend toward longer PFS (P = .0618; hazard ratio = 0.65). Objective response rate was significantly higher in the 175/75-mg group (22%) compared with the investigator's choice group (2%; P = .0019). Median overall survival for the temsirolimus 175/75-mg group and the investigator's choice group was 12.8 months and 9.7 months, respectively (P = .3519). The most frequent grade 3 or 4 adverse events in the temsirolimus groups were thrombocytopenia, anemia, neutropenia, and asthenia.

CONCLUSION

Temsirolimus 175 mg weekly for 3 weeks followed by 75 mg weekly significantly improved PFS and objective response rate compared with investigator's choice therapy in patients with relapsed or refractory MCL.

Expanding therapeutic options in mantle cell lymphoma

Mantle cell lymphoma (MCL) still carries a poor prognosis. Chemoimmunotherapy (combination with rituximab) is the routine first-line therapy, although data strongly suggest a benefit from intensification through high-dose therapy with stem cell transplantation consolidation or dose-intense chemotherapy with HyperCVAD (fractionated cyclophosphamide/vincristine/doxorubicin/dexamethasone)/rituximab. Unfortunately, most patients still experience relapse, and a multitude of novel agents are currently being tested in this setting, including proteasome inhibitors with bortezomib (the first of its class and first Food and Drug Administration-approved drug in MCL), mammalian target of rapamycin inhibitors, Bcl-2 inhibitors, and antiangiogenesis agents, among others. Because of the relative rarity of the disease-MCL represents 6% of non-Hodgkin lymphoma-an obvious effort is needed to enroll patients on clinical trials. Not surprisingly, as in other non-Hodgkin lymphomas, MCL appears more and more as a heterogeneous disease, which might impact future clinical trial design through pharmacogenomics and hopefully help us develop smaller "molecular" relevant trials.

Novel therapeutic targets in mantle cell lymphoma

Mantle cell lymphoma (MCL) is characterised by cell cycle dysregulation and a defective DNA damage response pathway. An evolving understanding of these processes has provided the rationale for development of novel agents targeting various steps that appear to be involved in lymphomagenesis and disease progression. Cyclin D1, overexpressed in nearly 100% of MCL, and the cyclin-dependent kinases were among the first rational targets identified. Therapies focusing on the PI3K/Akt pathway, the tumour microenvironment, and cell surface markers are also in various stages of exploration. Here, the authors discuss the rationale for developing targeted therapies and discuss future challenges in combining some of these agents.

The mammalian target of rapamycin signaling pathway: twists and turns in the road to cancer therapy

The immunosuppressive drug rapamycin played a key role in the functional characterization of mammalian target of rapamycin (mTOR), an unusual protein kinase that coordinates growth factor and nutrient availability with cell growth and proliferation. Several rapamycin-related compounds are now in various stages of clinical development as anticancer agents. This article highlights recent advances in our understanding of the mTOR signaling pathway and the implications of these findings for the clinical application of mTOR inhibitors in cancer patients.

Current development of mTOR inhibitors as anticancer agents

Mammalian target of rapamycin (mTOR) is a kinase that functions as a master switch between catabolic and anabolic metabolism and as such is a target for the design of anticancer agents. The most established mTOR inhibitors--rapamycin and its derivatives--showed long-lasting objective tumour responses in clinical trials, with CCI-779 being a first-in-class mTOR inhibitor that improved the survival of patients with advanced renal cell carcinoma. This heralded the beginning of extensive clinical programmes to further evaluate mTOR inhibitors in several tumour types. Here we review the clinical development of this drug class and look at future prospects for incorporating these agents into multitarget or multimodality strategies against cancer.