Inhibiting MEK in MAPK pathway-activated myeloma.

Over the last decade, new drugs have significantly changed the paradigm for treating multiple myeloma (MM), resulting in improved outcomes and reduced toxicity. However, many patients with MM relapse, and those who are refractory to or relapse after therapy with an immune-modulatory drug and a proteasome inhibitor have a dismal prognosis.[1] Improving the outcome of relapsed and refractory MM is a significant clinical challenge. Importantly, in this respect, recently published data have established the frequent mutation of the RAS/mitogen-activated protein kinase (MAPK) pathway,[2, 3, 4, 5] with mutations in NRAS, KRAS or BRAF being present in up to 50% of newly diagnosed MM cases. We routinely perform comprehensive genomic profiling using the FoundationOne Heme assay (Supplementary Methods). Review of these data shows the majority of the NRAS, KRAS and BRAF mutations occur in hotspots causing constitutive activation of the corresponding proteins. This makes the MAPK pathway a significant therapeutic target in MM.

Recent reports have demonstrated that MM cases with BRAF V600E mutations can respond to vemurafenib, even in the autologous stem cell transplant (ASCT) double-refractory setting, suggesting that blocking the MAPK pathway can be effective, even in end-stage, genetically complex cases.[6] Inhibition of BRAF using BRAF V600E inhibitors can result in paradoxical activation of the MAPK pathway, due to transactivation of CRAF,[7] a phenomenon that is exaggerated in KRAS-mutated cancers.[8] Inhibition of MAPK kinase (MEK) has emerged as a viable strategy to treat patients with BRAF-mutated cancers and to overcome paradoxical activation in the setting of therapy with BRAF V600E-directed agents. Trametinib is an oral, allosteric inhibitor of MEK1/2 that has shown early clinical activity in tumors with activating BRAF mutations. Preclinical studies have shown potent inhibition of MEK1/2 activation by preventing RAF-dependent phosphorylation of MEK.[9] Using trametinib in vitro resulted in inhibition of growth among most cancer cell lines and tumor xenografts, particularly those with activating mutations in BRAF or KRAS.[9]

As an index case of BRAF wild type, yet with an activating genomic alteration of the MAPK pathway, we report a case of a 52-year-old heavily pretreated man with MM who presented with treatment-resistant extramedullary disease (EMD). He was diagnosed with kappa light-chain MM in 2003, presenting with anemia, hypercalcemia and renal failure requiring hemodialysis. A detailed description of this patient's course of treatment and a timeline of events can be found in Supplementary Material and Supplementary Figure 1. He was initially treated with thalidomide and dexamethasone, followed by high-dose chemotherapy and ASCT. He relapsed in late 2005 with EMD in the liver and was treated with dexamethasone/cyclophosphamide/etoposide/cisplatin/thalidomide, resulting in a complete remission. In March 2006 he was treated with DT-PACE and tandem ASCT to consolidate his response, which was maintained with TD, keeping him disease free for 2 years. In December 2008 he relapsed with 84 FDG-avid focal bony lesions as well as EMD in the spleen and cervical lymph nodes. Evaluation of the bone marrow at that time showed 52% PC that were high risk by a gene expression profiling based 70-gene score (GEP70).[10] The patient underwent chemotherapy with PACMED (cisplatin, cytarabine, cyclophopshamide, mesna, etoposide, dexamethasone), resulting in a complete remission.

Between December 2008 and August 2013 the patient experienced multiple relapses and was treated with salvage therapies, which included ASCT, carfilzomib, pomalidomide, multi-agent chemotherapies, metronomic therapy and transarterial chemo-embolization, with varying degrees of responses. Over the course of his treatment, the patient developed EMD in the paraspinal muscles and the mesenteric lymph nodes in addition to treatment-resistant EMD of the liver.

In August 2013, comprehensive genomic profiling of CD138+ selected cells from his liver lesion using the FoundationOne assay revealed a KRAS Q61H mutation in 57% of cells. Four weeks after completion of his last salvage treatment at a time when there was positron emission tomography (PET)-proven persistence of disease, the patient was started on 2 mg trametinib daily. A follow-up PET 1 month later revealed complete resolution of all FDG avid lesions. Magnetic resonance imaging carried out 3 months after initiation of trametinib revealed complete resolution of previously identified liver lesion. In August 2014 Mekinist was stopped on account of a decreased left ventricular ejection fraction. The patient was noted to have relapsed disease by PET imaging and serum markers in October 2014.

To understand how this index case represents the RAS-mutated and MAPK pathway-activated population, we identified 58 additional patients who were treated with trametinib as a single agent or in combination with other drugs between August 2013 and May 2014 (Supplementary Figure 2). This retrospective review was approved by the UAMS institutional review board (IRB # 202984). All patients had provided informed consent. Electronic Medical Records and our Multiple Myeloma Data Base were reviewed to obtain demographic information, laboratory results as well as the patient's treatment history. Measurable disease was defined according to the International Myeloma Working Group.[11] For those patients with measurable disease, response was measured as the greatest percent change of measurable myeloma protein after initiation of therapy with trametinib. PET response was measured as the greatest percent change of number of FDG-avid focal lesions after initiation of therapy with trametinib. For the measurement of time on trametinib, drug holidays due to adverse events or for dose reduction were not considered as discontinuation of the drug. Lack of trametinib treatment for >3 weeks, that is, even if trametinib was added again at a later time point, was considered definite discontinuation.

Of the 58 patients, 51 patients were treated with trametinib based on the presence of oncogenic mutations of KRAS, NRAS or BRAF. Seven patients were treated based on GEP information suggesting an activation of the MAPK pathway.[12] The GEP information indicating overexpression of the MAPK pathway included overexpression of ITGB7, CCND2 or CCR1 (Supplementary Methods). Most patients had relapsed or refractory MM and received trametinib on an urgent basis, not allowing for a washout period. Their pre-trametinib features included cytogenetic abnormalities in 61%, while GEP70-defined high risk was present in 35%. PET scans available for all 58 patients showed medullary focal lesions in 30 cases (52%) and EMD in 11 (19%) (Supplementary Table 1). The median number of prior treatments was five, including Total Therapy trials[13, 14, 15] in 34 of 58 patients. Forty-two patients had at least one ASCT, 39 had salvage chemotherapy and 31 had been exposed to pomalidomide or carfilzomib.

Trametinib treatment was well tolerated. Of 58 patients treated, 24 discontinued therapy because of toxicities and 15 discontinued because of disease progression, physician's choice or death (Figure 1). The most significant adverse events were rash, diarrhea and cardiac toxicities. We observed 12 deaths. None of these was attributed to trametinib (Supplementary Table 2). Of the 58 patients treated with trametinib, 48 patients began treatment with monotherapy and 10 began with trametinib in combination with other agents (Supplementary Table 3). Of the 48 patients who began with trametinib as monotherapy, 26 had other agents added during the course of their treatment (Supplementary Table 4). Twenty-two patients received trametinib mono-therapy only (Figure 1).

Figure 1

Timeline of treatments and reasons for discontinuing therapy for 58 patients. Bar graphs represent days from start of treatment. Bar graphs show time of documented follow-up in aqua, time on multi-agent represented in green, time on single agent in blue, time since previous therapy in gray; discontinuation represented as a cross (X) following the color coding: adverse event in royal blue, progression in teal, physician's choice in yellow, other in pink; deaths are represented as diamonds in red.

Of the 40 patients with measurable disease at time of trametinib initiation, 23 patients experienced a reduction of the measurable MM protein by at least 25%. At least 50% reduction of the MM protein was seen in 16 patients (Figure 2a). This number was reduced to four when only considering the time on single agent trametinib (Supplementary Figure 3). Of the 24 patients with ⩾1 FDG-avid focal lesion on PET imaging at the beginning of treatment and available follow-up studies, 15 showed a >50% reduction in the number of focal lesions. Nine patients achieved complete remission based on positron emission tomography imaging (PET-CR) status, including six who had complete resolution of their focal lesions on single agent trametinib (Figure 2b and Supplementary Figure 4). In general, the PET response correlated well with a reduction of myeloma protein for most patients.

Figure 2

(a) Best response using protein criteria during treatment with trametinib as both single-agent and multi-agent therapies for 40 patients with measurable disease. Best response was determined as the greatest percent change in protein levels for patients with measurable disease. Measureable disease was determined using baseline test results and IMWG criteria, which requires one of the following: M protein (>1 g/dl), serum protein (>200 mg/24 h) or free light chain (involving FLC.10 mg/dl, and abnormal ratio). Patient response determined by free light chains are shown in pink, by urine protein shown in green, and serum protein shown in blue. (b) Best response by PET during treatment with trametinib for 24 patients. The bar graph shows patients with at least one focal lesion at baseline. Best response was calculated by the greatest percent change in number of focal lesions. Protein percent change was calculated by the greatest change by serum, urine or free light-chain values. PET results are shown in blue. Protein change is shown in pink.

At a median follow-up of 171 days, the median overall survival has not been reached, with 61% estimated to be alive at 260 days (Supplementary Figure 5). Due to the retrospective nature of this review an accurate estimate of progression-free survival (PFS) is not possible. We therefore used ‘time to next therapy' (TNT) as a surrogate for PFS. At a median follow-up of 171 days the median TNT was 186 days (95% confidence interval: 106–231 days) (Supplementary Figure 6).

Although this retrospective study may lack the patient uniformity afforded to clinical trials by stringent entry criteria and treatment protocol, it is more representative of the ‘real-life' patient population without bias toward benign disease features and better performance status. The trametinib single-drug response rate in a patient population in urgent need of therapy is reminiscent of our early investigations into thalidomide.

Trametinib shows promise as a myeloma therapeutic based on responses seen in this heavily pretreated MM population. The observation of complete responses with trametinib monotherapy supports the continued investigation of targeted therapy of the RAS/MAPK pathway and the use of trametinib as treatment for patients with activating MAPK pathway mutations who have exhausted standard treatments. A prospective trial evaluating the effect of trametinib on outcome in relapsed myeloma has been initiated.