Topical DNA Repair Enzyme May Prevent Skin Cancer
Topical DNA Repair Enzyme May Prevent Skin Cancer
FREEPORT, NYIn a phase III clinical trial of 30 patients with xeroderma pigmentosum, use of a topically applied DNA repair enzyme (T4N5 liposome lotion) for 1 year reduced the incidence of basal cell carcinoma by 30% and actinic keratoses by 68%, compared with placebo.
Xeroderma pigmentosum is a rare genetic disorder caused by defects in the repair of sunlight-induced DNA damage; patients with the disease are extremely sensitive to solar ultraviolet (UV) radiation and have high rates of skin cancers and actinic keratosis.
"Our focus has always been on DNA repair, and the most important outlet for DNA repair in oncology relates to skin cancer because this is the system in which we most clearly understand the carcinogenexposure to UV radiation," Daniel Yarosh, PhD, told ONI in an interview. Dr. Yarosh is the lead author of the trial and president of Applied Genetics Inc. (AGI) Dermatics, the company that developed T4N5 liposome lotion.
The lotion contains a bacterial DNA repair enzyme, T4 endonuclease V, encapsulated in a pH-sensitive engineered liposome for delivery into living cells. When delivered intracellularly, this enzyme increases the repair rate of sunlight-induced DNA damage in cells.
"We began 15 years ago," Dr. Yarosh said, "with the idea of applying DNA repair technology in skin cancer prevention." At that time, several enzymes capable of initiating DNA repair had already been cloned. "We chose T4 endonuclease V because it comes from a bacterial virus that binds specifically to the sites of dimers that are DNA damage," he said. "It makes a break at the site of damage in the DNA, and this initiates the repair process."
Studies of enzyme-initiated repair in bacteria have shown that using a microinjection technique to insert the enzyme into human cells increases the repair process. The major barrier, however, was finding a nontoxic method to deliver the repair enzymes into the cells of the skin.
The researchers decided upon liposomes, since they are composed of phospholipids that are very similar to the phospholipids in the keratinocyte, the cell that gives rise to many forms of skin cancer.
"We packed the enzyme into this liposome and engineered it to penetrate the skin, be taken up by the keratinocytes, and then open up and deliver the contents. This way the enzyme only penetrates the skin cells," Dr. Yarosh said. "Since the liposomes are localized to the epidermis, there’s no systemic delivery and consequently no allergic reaction. There couldn’t be a more benign route of administration."
In phase II clinical studies of patients with xeroderma pigmentosum, T4N5 liposome lotion was applied to sites of UV irradiation; after 6 hours, a specimen was taken for biopsy. Then, using a monoclonal antibody to identify and assess DNA-damaged cells, the amount of damage remaining in the treated sites vs the untreated sites was compared.
"These phase II studies showed that we could increase the amount of repair by about 20% in just 6 hours, compared with control," Dr. Yarosh said.
At the same time, the company did similar tests in patients previously treated for skin cancer. These patients received UV radiation, followed by treatment with the T4N5 liposome lotion. "These patients also showed a similar increase in DNA repair," he said.
After reviewing these data, the company did a prospective, multicenter, randomized, double-blind phase III study in 30 xeroderma pigmentosum patients with a history of actinic keratosis or skin cancer. Twenty patients were assigned to treatment and 10 to placebo (2 of the placebo patients withdrew from the study before completion).
The liposome lotion was applied daily to the arms and face for 1 year; the subjects also used sunscreen. They were examined every 3 months, and their skin cancers were counted and removed.
"We observed a 30% reduction in basal cell carcinoma, which is the most common form of skin cancer, with approximately 1 million new cases presenting in the United States every year," Dr. Yarosh said. The mean rate of new basal cell car-cinomas was 5.4 per patient per year in the placebo group vs 3.8 in the treatment group.
Actinic keratoses were reduced by 68% with use of T4N5 liposome lotion. The mean annual rate of new actinic keratoses was 8.2 in the treatment patients vs 25.9 in the placebo group. The rates of development of squamous cell carcinoma and melanoma were not large enough for statistical analysis. (For a complete report, see Lancet 357:926-929, 2001.)
Dr. Yarosh said that no adverse effects were seen during treatment, and no antibodies against the enzyme were found in patients’ serum.
Although the drug showed efficacy in reducing DNA damage in conditions such as actinic keratosis, which is thought to be a precursor to squamous cell skin cancer, there is still a controversy over whether actinic keratosis is actually a premalignant condition. Therefore, Dr. Yarosh said, the FDA drug application for approval of the lotion will focus on the basal cell carcinoma results.
He noted that the larger issue of preventing skin cancer in the vast population of high-risk individuals who do not have xeroderma pigmentosum is the basis for future studies.
"People who have already had one skin cancer have at least a 25% chance of developing another skin cancer within 5 years. Also, fair-skinned, light-haired individuals who sunburn easily are at increased risk of developing skin cancer. In both of these populations, the issue of prevention looms large," Dr. Yarosh said.
To objectively determine if the drug will prevent cancer in people without a genetic defect, the company plans to do a large population-based study. Some 600 people will be followed over a period of 1 year. All the people enrolled in the study will be fair skinned and light haired, and live in the same geographical area. Levels of UV exposure will be equivalent to that found in Lubbock, Texas, he said.
The researchers selected Lubbock as a model for the geographic location for the study (which has not yet been selected) because, Dr. Yarosh said, "there was a very good study done there looking at sunscreen in prevention of actinic keratoses. So the demographics have already been worked out, and we know the standard deviation of the group we’ll be enrolling."
From the results of several clinical trials in xeroderma pigmentosum patients, it is evident that the application of liposomal DNA repair enzymes to sun-damaged skin reduces the incidence of at least some forms of skin lesions.
"Given that knowledge," Dr. Yarosh concluded, "we feel that T4N5 liposome lotion also holds promise for people who have already had skin cancer and, ultimately, as a preventive measure for those who have never had skin cancer. Naturally, we recognize that there needs to be further testing of the safety and efficacy of T4N5 liposome lotion before it can be recommended for use as a cancer preventive in the general population."