Vemurafenib: a milestone in personalised medicine

The recent development of agents such as vemurafenib (Zelboraf) is offering an alternative to conventional chemotherapeutic agents and personalising the face of treatment for metastatic melanoma.

Malignant melanoma is the seventh most common cancer in men in the UK and the sixth most common in women.(1) In 2009, there were approximately 11,877 new diagnoses of malignant melanoma and, in 2010, 2203 deaths were registered in the UK alone.(1) Diagnosis occurs at a mean age of 50 years with approximately 20% of cases occurring in young adults between 15 and 39 years of age.(2) Prognosis for patients with metastatic melanoma is poor and treatment options are limited, particularly for second or subsequent lines of therapy. Treatment with dacarbazine, the chemotherapeutic agent considered to be the ‘gold standard’ for most patients, results in an overall survival of between 5.6 and 7.7 months.(3–5)

Personalised medicine

Personalised medicine is a concept that healthcare professionals and the pharmaceutical industry have been working towards for many years. The fundamental aim is to tailor patients’ treatment based on the genetics of their disease to maximise efficacy while minimising toxicity. This model also encourages more cost-effective use of resources by ensuring that these treatments are directed towards those who are known to potentially gain the most benefit from them. Researchers have identified numerous genetic variations that might play a role in the pathogenesis of cancer. These variations provide perfect targets for the pharmaceutical industry to use as a focus for drug development. However, where breakthroughs are made in this way, effective and readily available diagnostic tests are required to ensure that the use of such drugs will be effective in clinical practice. There are many examples where this combination of targeted therapy, facilitated by specific diagnostic tests, have become established treatments, routinely used in practices throughout the world, for example, trastuzumab for HER2+ breast cancer and cetuximab for KRAS mutant colorectal cancer.

MAPK pathway and BRAF

More recently, the mitogen-activated protein kinase (MAPK) pathway has been identified as having a significant role in the development of melanoma (Figure 1). The serine/threonine kinase, BRAF, is an integral part of this pathway. Activation results in cell proliferation and survival. Mutations in the BRAF protein can lead to increased kinase activity and activation of its downstream targets.(6) The most common mutation seen is the substitution of valine 600 to glutamic acid. The resulting protein is referred to as BRAF V600E and this mutation is present in approximately 40–60% of melanomas.(7)

Vemurafenib

Vemurafenib (Zelboraf), a first-in-class inhibitor of mutant BRAF, was licensed in Europe in 2012 as monotherapy for the treatment of adult patients with BRAF V600 mutation-positive unresectable or metastatic melanoma.

Preclinical and current data suggest that the activity of vemurafenib is minimal in tumours lacking BRAF mutations; therefore, it is crucial to have a robust and accurate BRAF mutation assay to enable effective selection.(6) Current methods of BRAF mutation testing are variable in their level of validation and methods. Direct (Sanger) sequencing, for example, demonstrates limited sensitivity in specimens containing low percentages of mutations.(7)

The cobas® 4800 BRAF V600 mutation test was developed in parallel with vemurafenib as a qualitative, molecular assay designed to be used in selecting melanoma patients whose tumours carry the BRAF V600E mutation,(8) The cobas® 4800 BRAF V600 mutation test is based on two processes designed to enable preparation of the specimen: DNA is extracted from formalin-fixed, paraffin-embedded human tissue (FFPET); and real-time detection (polymerase chain reaction (PCR) amplification) of the wild-type BRAF V600 sequence and V600E mutation.(8,9)

The test is designed to detect the nucleotide (T1799A) change in the BRAF gene that results in a valine-to-glutamic acid substitution at codon 600 (V600E). BRAF wild-type and mutant DNA target-specific fluorescent dye-labelled TaqMan probes bind to the wild-type and mutant sequences, respectively.

The wild-type and mutant sequences are detected using a dedicated optical channel for each sequence.(8,9) Lopez-Rios et al(10) conducted a study comparing molecular testing methods of real-time PCR assay (cobas® test) to two commercially available methods: Applied Biosystems’ BRAF Mutation Analysis Reagents (ABI), and 2x bidirectional Sanger sequencing (Sanger). Testing for the detection of BRAF V600E mutations was conducted in 116 FFPET specimens of malignant melanoma. Invalid results were present in 8/116 (6.9%) with Sanger, 10/116 (8.6%) with ABI and 0/232 (0%) with cobas®.

Hit rates for 5% mutation blends were 100% for the cobas® test, 33% for Sanger and 20.8% for ABI. Turnaround times were approximately five days for Sanger, two days for ABI and one day for the cobas® test. The study concluded that the cobas® test was highly reproducible and had the lowest invalidity rate, as well as greater sensitivity than either Sanger or ABI and a more rapid turnaround time.

Roche Pharma and Roche Molecular Diagnostics have worked together to support the provision of a free BRAF V600E mutation detection service. A patient is eligible for this service if:

Key clinical trials

Pivotal trials in establishing the data to support the effectiveness of vemurafenib are centred on the BRIM-2 and BRIM-3 studies.(11,12) The BRIM-2 study was an open-label, multi-centre, phase II study in patients with previously treated melanoma who tested positive for the BRAF V600E mutation. A total of 132 patients were recruited with the primary endpoint as best overall response rate. The results demonstrated a best overall response rate of 52.3% (95% CI 43–61%), with a median progression-free survival (PFS) of 6.2 months.(11) Subsequently, the phase III BRIM-3 study was conducted to compare vemurafenib with dacarbazine in previously untreated BRAF V600E-positive metastatic melanoma patients. A total of 675 patients were randomised to receive either oral vemurafenib 960mg twice daily (n=336) or dacarbazine 1000mg/m2 (n=336) intravenously every three weeks.

Co-primary endpoints were rates of overall survival and PFS. At six months, the relative risk of death was reduced by 63% and the risk of death or progression reduced by 74% (both p<0.001). Overall survival was 84% (95% CI 78–89) in the patients treated with vemurafenib compared with 64% (95% CI 56–73) in those receiving dacarbazine. PFS was 5.3 months and 1.6 months in the vemurafenib and dacarbazine arms, respectively. This improvement was seen across all subgroups including age, gender and disease stage. In the vemurafenib group, 48% of patients had a confirmed objective response (with two complete and 104 partial responses). Only 5% of patients in the dacarbazine arm had a confirmed response (all partial).(12) More recently, a cut-off of the results has shown that median overall survival was 13.2 months and 9.6 months for vemurafenib and dacarbazine, respectively.(13) Table 1 summarises the results.

Adverse effects

Common adverse events in the vemurafenib arm (reported as percentage of patients experiencing grade 2–3) included arthralgia (21%), rash (18%), fatigue (13%) and photosensitivity (>10%). Other reported adverse events include alopecia, nausea and pruritis.(12)

Photosensitivity is particularly problematic and patients should be advised to avoid exposure to sunlight. Where this is not practical, protective clothing and broad-spectrum UVA and UVB sun cream and lip balms should be used. Vemurafenib therapy has also been associated with the development of squamous cell carcinoma. In this study, 12% of patients being treated with vemurafenib experienced this. Other studies have found the incidence to be >20%(12,13) The vast majority of these lesions can be managed by excision, but approximately 33% will recur. The precise mechanism for this paradoxical increase in tumour growth is not yet clear and investigations are ongoing.(12) For this reason, patients should be monitored closely during and after treatment with vemurafenib for development of such lesions.

Resistance 

Resistance to vemurafenib has been observed and investigations are ongoing to identify the mechanisms involved. Resistance is well documented with other kinase inhibitors and often results from a mutation that prevents the drug from binding to the target site effectively. In the case of chronic myelogenous leukemia, resistance to imatinib has been overcome by subsequent development of second- and third-generation agents.

Conclusions

Vemurafenib and the recently licensed monoclonal antibody for CTLA4, ipilimumab, represent the most dramatic advances in the management of a disease that has had limited treatment options for several years. Other BRAF inhibitors, such as dabrafenib, have shown promise in phase III clinical trials in addition to novel agents acting on the MAPK pathway. The sequence in which these agents are used is also an area of discussion, as optimisation may lead to prolongation of survival. The opportunity for discovering further molecular targets means that this is certainly not the end of developments in this area and there is reason to be optimistic about improving the prognosis for these patients further in years to come.

Key points

• Vemurafenib (Zelboraf) is an inhibitor of BRAF V600E that has been shown to improve overall survival in malignant melanoma compared with treatment with dacarbazine.
• Mutant BRAF (V600E) is found in 40–60% of melanomas. This results in constitutive activation of BRAF and its downstream targets, leading to increased cellular proliferation and survival.
• The cobas® 4800 BRAF V600 mutation test was developed in parallel with vemurafenib
• as a qualitative, molecular assay designed to be used in selecting melanoma patients whose tumours carry the BRAF V600E mutation.
• Patients should be monitored for development of squamous cell carcinoma during vemurafenib treatment and advised regarding management of photosensitivity.
• Vemurafenib is a milestone in personalised medicine, demonstrating how tailoring a patient’s treatment based on the genetics of his/her disease can maximise efficacy while minimising toxicity.

Authors

Christopher Watson MPharm
Champa Mistry MPharm MRPharmS
Saadhiya Hussain MPharm

Pharmacy Department, St Bartholomew’s Hospital, London, UK

Email: [email protected]

Acknowledgments