Spectrum Of Therapies
Melanoma accounts for less than 1% of skin cancer cases yet causes the vast majority of skin cancer deaths. If detected early enough, melanoma is almost always curable. If not, its ability to metastasize makes it difficult to treat.
Melanoma is more common in young adults than many other types of cancer, with 25% of new cases occurring in people under age 45. Its prevalence is growing — the number of new cases per year relative to the total population has doubled since 1973. In 2017, there will be an estimated 87,110 new cases of melanoma in the U.S. and 9,730 melanoma-related deaths, according to the Aim at Melanoma Foundation. Let’s review the basics and find out the latest treatments in the battle against melanoma.
Melanoma is the uncontrolled growth of melanocytes, the pigment-producing cells located in the bottom layer of the skin’s outermost layer (the epidermis). In skin cancer, a tumor initially grows and spreads within the epidermis due to DNA damage, which is usually caused by ultraviolet (UV) radiation.
If melanoma is detected during the epidermal level growth stage, it can often be surgically removed. Penetrating the deeper layers of the skin as it grows, it will eventually come into contact with lymph and blood vessels — which act as a cancer highway. When melanoma spreads to other parts of the body it is known as metastatic melanoma.
Lighter-skinned people are at higher risk for melanoma because the increased skin pigmentation found in darker skin tones helps to block UV rays from penetrating and damaging skin cell DNA. However, darker-skinned people can and do get skin cancer. Another general risk factor for melanoma is atypical moles, or moles with irregular shape, color, or size. Moles are clusters of melanocytes and so sudden changes in them may be an early warning sign of melanoma.
Genetic Factors: p53 & BRAF
Although most cases of skin cancer are traceable to excessive UV radiation from sun exposure, about 10% are likely due to genetic factors. The gene most commonly mutated in familial melanoma is p53. p53 is a “tumor suppressor” — it detects DNA damage in cells and triggers either DNA repair pathways or activates cell death if the damage cannot be repaired. Another gene, known as the BRAF gene, regulates cell growth and is mutated in inherited forms of melanoma. About half of all genetically-based melanomas have the BRAF mutation.
Let’s take a closer look at BRAF. BRAF codes for a protein required for the transmission of a growth signal from a cell surface receptor to the cell nucleus (growth signal transduction). Growth signaling is initiated by a growth factor binding to its receptor. This binding transmits a signal through the membrane, causing the internal portion of the receptor to interact with and activate a protein inside of the cell. This activation is then transferred to the next protein in the pathway, and so on until the signal reaches the last protein in the pathway. When this protein is activated, it enters the nucleus, where it turns on specific genes that make proteins which initiate cell division. BRAF is one of the proteins in this pathway. In BRAF-associated melanoma, the mutated BRAF is always turned on even when no growth factor is present.
The small molecule drugs Zelboraf (Genentech; South San Francisco, CA) and Tafinlar (Novartis; Basel, Switzerland) inhibit overactive BRAF and are approved for the treatment of late-stage melanoma.
Immunotherapies In The Fight
A few different checkpoint inhibitor therapies are on the market for metastatic melanoma. These therapies enable killer T-cells – immune system cells that recognize and kill threats such as cancer cells – to become fully activated and able to kill tumor cells. Keytruda (Merck, Kenilworth, NJ) and Opdivo (Bristol-Myers Squibb, New York, NY) both target PD-1, an inhibitory protein on the surface of T-cells; Yervoy (Bristol-Myers Squibb) targets a second inhibitory protein, CTLA-4. Both PD-1 and CTLA4 essentially act as “off switches” for killer T-cells. By inhibiting these off switches, the killer T-cells become fully activated, and able to kill melanoma cells.
A second type of immunotherapy approved for melanoma is Amgen’s (Thousand Oaks, CA) oncolytic virus therapy, talimogene laherparepvec (T-VEC). An oncolytic virus is a virus that infects and kills cancer cells. The cancer cells are killed through cell lysis: as the virus multiplies inside of the cells, it causes them to burst open. This in turn releases new infectious particles that can target remaining tumor cells. In addition to killing cancer via lysis, the presence of an actively replicating virus triggers the immune response to target the tumor. T-VEC was designed to infect and kill melanoma cells, and is so far the only FDA-approved oncolytic virus.
Incyte (Alapocas, Delaware) has a new type of small molecule immunotherapy in Phase III called Epacadostat – it inhibits the enzyme IDO1. IDO1 helps regulatory T-cells to develop and become activated. Regulatory T-cells suppress the immune response, and therefore help cancer cells to escape immune surveillance. Inhibiting IDO1 should suppress the development of regulatory T-cells, bolstering the immune response against melanoma.
The Implications Of MicroRNA
Researchers at Tel Aviv University examined how melanoma metastasizes in an article last summer: the melanoma cells release tiny vesicles containing microRNA, a type of regulatory RNA produced by all cells.
The microRNA-filled vesicles induce changes in the layer of skin just below the epidermis, where melanoma originates. That lower layer of skin, known as the dermis, contains blood vessels which cancer cells are able to access. The Tel Aviv team is identifying drug candidates that may interfere with this process, preventing the metastasis that makes melanoma so deadly.
Increased understanding of the molecular pathways that contribute to melanoma’s development and spread will provide additional tools to fight those cases of metastatic melanoma that inevitably will continue to arise.
Emily Burke, PhD has worked in biopharma for 20 years, gaining science writing experience at The Scripps Research Institute and Ionis Pharmaceuticals. As a Ph.D. molecular biologist, she is passionate about advancing the public’s understanding of science. In addition to being a self-proclaimed “science geek,” she is regularly asked to speak at international scientific meetings. When not teaching and writing the WEEKLY for Biotech Primer, Dr. Burke swims with her swim club and performs regularly on the improv circuit in San Diego.