Initially approved in 2012, vismodegib is the first FDA-approved pharmacologic agent that targets the Hedgehog signaling pathway (Sonic Hedgehog, SHH), a pathway involved in many basal cell carcinomas. Basal cell carcinoma (BCC) is the most common nonmelanoma skin cancer (NMSC), accounting for over one-half of all NMSC diagnoses. BCC affects over 3.3 million people a year in the United States. Approximately 40% of patients diagnosed with BCC will develop another BCC within five years of the initial diagnosis. Management is primarily with surgical intervention, except in some cases that utilize topical therapy. Not all patients are suitable candidates for surgery or topical therapy due to locally advanced or metastatic basal cell carcinoma; this is when vismodegib has a place in therapy.
Vismodegib indications include the following:
Vismodegib first received approval after the results of the ERIVANCE study, which showed results of treating locally or advanced metastatic basal cell carcinoma in situations where simple excision would not result in a cure. The ERIVANCE study showed a response rate of 45% in those that had locally advanced disease. In patients with metastatic basal cell carcinoma, an independently assessed response rate was 30%. A systematic review showed that patients with locally advanced basal cell carcinoma had a complete response rate in the range of 0% to 54.1%, with a weighted average of 31.1%. Partial response rates for those with locally advanced disease were between 25.5% and 66.7%, with a weighted average of 33.6%. A partial response indicated greater than 30% reduction in total tumor size, and a complete response rate indicated that within subsequent biopsies, no residual basal cell carcinoma tumor was present.
Recently, researchers studied vismodegib as neoadjuvant therapy. In an open-label clinical trial of 15 patients using vismodegib as a neoadjuvant treatment before surgery in high-risk basal cell carcinoma, there was a decrease in the final surgical defect size by 34.8% with no recurrence, and no skipped areas were present in the resectioned tumor blocks. The study was small, and only 11 patients completed therapy with vismodegib. However, future studies may examine this topic further.
The Hedgehog pathway regulates cell growth and differentiation in embryogenesis. This pathway is not active in adult tissue. PTCH1 is a tumor suppressor gene involved in the SHH pathway. SMO is a G-protein-coupled transmembrane receptor involved in signal transduction of the SHH pathway. Mutations in the smoothened (SMO) and PTCH1 genes appear to lead to abnormal up-regulation and activation of the SHH signaling pathway. Protein patched homolog 1 (Patched) is produced by the gene PTCH1 and functions as a transmembrane receptor that, when activated, suppresses the migration of the SMO receptor to the cell membrane. When the SHH ligand binds to Patched, the SMO receptor is released into the cell membrane, after which activation of cell proliferation occurs. In basal cell carcinoma, mutations in the Hedgehog pathway can result in an unrestricted proliferation of basal cells; researchers have noted that this pathway is relevant in more than 90% of basal cell carcinoma cases. One of the mechanisms believed to be involved in tumorigenesis involves ligand-independent pathway activation by SMO.
Vismodegib is a selective Hedgehog pathway inhibitor that binds to and competitively inhibits SMO. Inhibition of SMO results in transcription factors GLI1 and GLI2 remaining inactive; this prevents the expression of tumor-mediating genes within the Hedgehog pathway.
Mutations in PTCH1 have correlations with nevoid basal cell carcinoma syndrome (Gorlin syndrome), trichoepitheliomas, squamous cell carcinoma of the esophagus, transitional cell carcinoma of the bladder, and holoprosencephaly. Medulloblastoma is a nervous system tumor associated with Hedgehog pathway mutations in as much as 30% of cases.
Vismodegib has a bioavailability of approximately 32% and has time-to-peak levels of approximately 2.4 days. Greater than 98% of the drug circulates as the original compound. Once absorbed, it has a volume of distribution (Vd) between 16 and 26.6 liters and is highly protein-bound (greater than 99%) to serum albumin and alpha acid glycoprotein (AAG). Vismodegib undergoes metabolism by oxidation, glucuronidation, and ring cleavage. It is a minor substrate for CYP2C9 and CYP3A4. The half-life of elimination after a single dose is approximately 12 days, but with continuous daily dosing, it becomes four days before it is eliminated primarily in the feces (82%) and urine (4%).
It is also noteworthy that vismodegib concentrates in the semen of men.
Vismodegib is available as a 150-mg oral-use capsule. It is dosed at 150-mg orally once daily until disease progression or unacceptable toxicity.
However, adverse effects associated with vismodegib may serve as a barrier to lifelong treatment. Recently in an article published in Lancet Oncology, intermittent dosing protocols were examined over 73 weeks comparing two groups with different dosing, Group A and Group B. Group A received 12 weeks of vismodegib followed by eight weeks of placebo, which was repeated until the 73-week mark. Group B was given vismodegib initially for 24 weeks, followed by eight weeks of placebo, and then alternating eight weeks of vismodegib with eight weeks of placebo until the 73-week mark.
Both groups A and B had a reduction in the mean number of basal cell carcinoma lesions from baseline (62.7% and 54%, respectively). However, the number of treatment-emergent events was higher in group B. Treatment activity and adherence were also lower within group B when compared to group A. It is likely that researchers will examine different dosing protocols going forward to balance adverse effects with treatment effectiveness.
The adverse effects of vismodegib have been a limiting factor for the medication. All patients in the ERIVANCE trial experienced at least one adverse event. In the ERIVANCE trial, 57% had a mild (grade 1) or moderate (grade 2) adverse event. A total of 13 individuals (12%) had an adverse event that ultimately led to the discontinuation of vismodegib.
The most common adverse effects include muscle spasm (72%), fatigue (40%), alopecia (64%), dysgeusia (55%), and weight loss (45%). Other adverse effects associated with vismodegib include nausea, diarrhea, constipation, decreased appetite, ageusia, and arthralgias. Some patients may experience electrolyte disturbances, including hypokalemia or hyponatremia. Additionally, there may be elevated creatine phosphokinase levels and azotemia.
Recently, researchers at the University of California in San Francisco evaluated vismodegib to see if there was a risk in the development of subsequent squamous cell carcinoma following use. A retrospective cohort study determined that vismodegib was not associated with an increased risk of SCC compared to those with standard surgical treatment for basal cell carcinoma. However, further studies are likely to examine this topic.
Vismodegib is contraindicated in pregnant women or women who may become pregnant.
Boxed Warning of Embryofetal Toxicity
Embryotoxic effects include:
Patients should be advised not to donate blood or blood products during vismodegib treatment and for at least 24 months after the last dose.
Important Special Warnings
Counseling of both males and females on vismodegib is necessary.
Following a negative pregnancy test, initiate highly effective contraception before the first dose of vismodegib and continue for seven months after treatment.
During treatment, and for three months following treatment, men should not donate sperm and should use condoms with spermicide (even after vasectomy) as vismodegib may be present in seminal fluid. This precaution is to avoid exposure to patients that may be pregnant.
Pregnancy test within one week before treatment initiation.
The prescription of vismodegib is limited to the oncologist and dermatologist. However, the pharmacist also needs to know the basic principles of this agent. Initially approved in 2012, vismodegib is the first FDA-approved pharmacologic agent that targets the Hedgehog signaling pathway (sonic hedgehog, SHH), a pathway involved in many basal cell carcinomas. Not all patients are suitable candidates for surgery or topical therapy due to locally advanced or metastatic basal cell carcinoma; this is when vismodegib has a place in treatment.
The adverse effects of vismodegib have been a limiting factor for the medication. The most common adverse effects include muscle spasm (72%), fatigue (40%), alopecia (64%), dysgeusia (55%), and weight loss (45%). There is renewed concern that the drug may induce the development of squamous cell cancer. Hence, all patients prescribed vismodegib require close surveillance.
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