Thalidomide and its analogues in the treatment of Multiple Myeloma

Thalidomide was first introduced as an oral sedative and anti-emetic in 1957, but was quickly abandoned due to its profound teratogenic effects. Almost four decades passed before studies demonstrated thalidomide to have anti-cancer properties, specifically showing significant in vitro anti-myeloma activity. It found its use as a novel anti-myeloma drug for relapsed and refractory disease in 1999 [8], later yielding impressive overall response rates of up to 50% when used in combination with dexamethasone and up to 65% when combined with corticosteroids and cyclophosphamide [9]. Thalidomide, though effective, was associated with dose-limiting toxicities including somnolence, constipation, neuropathy [10], and increased incidence of venothromboembolism (VTE), especially when combined with dexamethasone [11]. Hence, a new class of thalidomide derivatives called IMiDs was developed that, albeit structurally related (Figure 2, Table 1), had their unique set of anti-inflammatory, immunomodulatory, antiproliferative, antiangiogenic and toxicity profiles [12].

Lenalidomide (CC 5013) was FDA approved for use in patients with relapsed and/or refractory multiple myeloma after successful clinical trials [13, 14]. Lenalidomide has a distinct adverse effect profile compared to thalidomide. Notably, there is an increased risk of grade 3, or greater myelosuppression; however, the rates of sedation, constipation and neuropathy are all lower than thalidomide. The risk of VTE varies among studies, and is dependent on age, concurrent chemotherapy including dexamethasone, and the use of VTE prophylactic agents. Importantly, lenalidomide adversely affects adequate peripheral blood stem cell collection in older patients with therapy duration greater than 4 months [15, 16]. More recently, several investigators have reported an alarmingly high rate of second malignancies, both myelodysplastic syndrome/acute myeloid leukemia and solid tumors in newly diagnosed patients with multiple myeloma receiving maintenance lenalidomide after autologous stem cell transplant (Table 2) [17‐19]. However, additional data reported in 2011 Annual Meeting of American Society of Clinical Oncology did not show any increase in the rate of second malignancies in transplant ineligible and relapse or refractory patients undergoing lenalidomide therapy [20‐22]. For example pooled data from MM-009/010 trials evaluating lenalidomide and dexamethasone versus dexamethasone/placebo and a phase II trial evaluating clarithromycin, dexamethasone and lenalidomide in 72 newly diagnosed multiple myeloma patients found similar frequency of second malignancies to background incidence and 2010 SEER data respectively for non-Multiple Myeloma individuals of similar age.

Pomalidomide (CC-4047), another derivative of thalidomide has shown promising activity in phase I/II trials in relapsed and refractory multiple myeloma patients, including patients who have been treated with thalidomide, lenalidomide and the 26S proteasome inhibitor bortezomib (PS-341) [23‐26]. Overall pomalidomide was well tolerated, with thromboembolic events the major non-hematological dose limiting toxicity, while neutropenia was the most common dose-limiting toxicity noted in initial phase 1 study. Peripheral neuropathy was reported by up to 30% of patients; however, the majority of these were reversible grade 1toxicity [23, 24].

The mechanism of action of the IMiDs is not completely understood. They are thought to possess anti-proliferative, anti-angiogenic and immunomodulatory effects against myeloma cells. Lenalidomide and pomalidomide exert their anti-myeloma effect dually through direct down-regulation of key functions of the tumor cell, and indirectly by modulating interaction of myeloma cells with their microenvironment. They carry out their direct effects via modulation of cell adhesion and decreasing production of key pro-survival cytokines such as TNFalpha, IL-6, IL-8, and VEGF that favor tumor cell survival and proliferation, inhibition of apoptosis, and resistance to therapy [27].

Both lenalidomide and thalidomide inhibit TNFalpha production by increasing the degradation of TNFalpha mRNA and by increasing the activity of alpha-1-acid glycoproteins, which have intrinsic anti-TNFalpha activity [28]. In addition to the anti-angiogenic effect of thalidomide, IMiDs have further shown to directly induce apoptosis by triggering the activation of caspase-8, which in turn induces myeloma cell death via Fas-mediated pathways. Dexamethasone also activates caspase-9 pathway, hence forming the basis of its synergy with the IMiDs. Lenalidomide also utilizes mitochondrial pathways to release pro-apoptotic proteins such as cytochrome c and Smac [29]. Dexamethasone too, has been shown to release Smac, but not cytochrome c, in myeloma cells, hence providing further molecular basis for the additive effects of the two drugs, as seen in clinical trials.

IMiDs have shown to down-regulate NF-κB transcription activity in vitro, and in effect blunt out the survival advantage that myeloma cells employ in tumorigenesis. Dexamethasone has also been shown to down regulate NF-κB activity, and coupled with lenalidomide nearly abrogates NF-κB activity in myeloma cells [30]. The net effect is inhibition of caspase inhibitors, induction of caspase-8 activity, and promotion of Fas- and TRAIL/Apo2L- induced apoptosis. IMiDs also block the stimulatory effect of insulin like growth-factor-1 on NF-κB activity and potentiate the activity of dexamethasone and proteasome inhibitor therapy.

Furthering their immune modulatory role, the IMiDs also affects other cellular components in the bone marrow. In preclinical studies they have demonstrated activation of T and NK cells [31‐33]. Lenalidomide induces T-cell proliferation, mainly by co-stimulation of CD28 that in turn leads to increased transcription and production of IL-2 and IFN-gamma. It is in fact many times more potent than its predecessor. As a result of increased T-cell derived IL-2 and IFN-gamma, and lenalidomide and pomalidomide have been shown to augment CD 56+ NK cell numbers and function [34].

In addition to its diverse immunomodulatory effects, lenalidomide, although a weaker inhibitor of angiogenesis than thalidomide, has a well-documented anti-angiogenic profile. Instead of directly inhibiting endothelial cell proliferation like its predecessor, lenalidomide in fact inhibits the secretion of vascular endothelial growth factor (VEGF) and basic fibroblast growth factor (bFGF), from both tumor and stromal cells [35]. Lenalidomide inhibits Akt phosphorylation, hence interrupting the phosphatidylinositol 3-kinase (PI3K)/Akt signaling pathway, known to be integral in tumor cell survival and proliferation. Inhibition of this pathway, downstream of VEGF signaling may explain in part the drug’s anti-proliferative and anti-angiogenic effects. Lenalidomide also inhibits VEGF-induced PI3K-Akt pathway signaling, which is known to regulate adherens junction formation. A strong inhibitory effect of lenalidomide on hypoxia-induced endothelial cell formation of cords and hypoxia-inducible factors (HIF-1) alpha expression, the main mediator of hypoxia-mediated effects and a key driver of angiogenesis and metastasis, has also been found [36]. Pomalidomide and lenalidomide also demonstrated their potential in epigenetic modulation of p21 WAF-1 expression, thereby possibly being efficacious in p53 mutated tumors [37].

Recent landmark discovery of cereblon (CRBN), an essential protein to induce thalidomide teratogenicity in zebrafish and chicken embryos, led to several presentations at 2011 American society of Hematology (ASH) meeting demonstrating the critical role of CRBN in anti-proliferative response to IMiDs [38, 39]. Majority of myeloma cell lines and patients resistant to IMiDs showed very low levels of CRBN [40]. In addition, bone marrow CRBN mRNA expression was also predictive of response to lenalidomide and dexamethasone in small study of 44 patients [41].