Nonsurgical Innovations in the Treatment of Nonmelanoma Skin Cancer

Abstract

Basal cell carcinoma and squamous cell carcinoma are the most frequent types of cancer in the United States and represent 75 percent and 20 percent, respectively, of all nonmelanoma skin cancers. Since ultraviolet radiation is implicated in their development, photoprotection is fundamental in their prevention. Additional preventive measures include identifying high-risk individuals for early detection along with using agents, such as retinoids, that are effective in decreasing the risk of premalignant cells further developing into carcinomas. Newer agents achieving this goal include perillyl alcohol, T4 endonuclease 5, DL-α-tocopherol, and α-difluoromethylornithine. Procedural modalities are currently the standard of treatment, but recent evidence has consistently shown that newer (nonsurgical) therapies, such as interferon, imiquimod, retinoids, and 5-fluorouracil, can be used effectively either as monotherapies or as adjuvants to those surgical modalities for the treatment of superficial nonmelanoma skin cancers and premalignant lesions. These newer therapies have achieved significant reductions in morbidity and mortality. Procedural modalities that have been evolving into important tools for the treatment of actinic keratosis and nonmelanoma skin cancers include photodynamic therapy and lasers. Nonsurgical therapies currently proving to be effective in clinical trials include ingenol mebutate and cyclooxygenase-2 inhibitors. Agents that are showing promising results in early phases of clinical trials include betulinic acid; hedgehog signaling pathway inhibitors, such as cyclopamine and GDC-0449; α-melanocyte–stimulating hormone analogs, such as afamelanotide; epidermal growth factor receptor inhibitors, such as gefitinib and erlotinib; anti-epidermal growth factor receptor monoclonal antibodies, such as cetuximab and panitumumab; and the 5-fluorouracil prodrug capecitabine.

Nonmelanoma skin cancer (NMSC) represents the most common form of cancer in humans, with an estimate of more than 1,000,000 new cases and 1,000 deaths in the United States in 2009.^1–3 The two subtypes associated with ultraviolet radiation (UVR) as a major contributory factor, basal cell carcinoma (BCC) and squamous cell carcinoma (SCC), account for 75 percent and 20 percent of the cases, respectively.^2,4,5 Although the relative mortality is low (0.1%), NMSCs may cause considerable morbidity, particularly in visible areas, such as the head and neck, with consequent unacceptable cosmetic outcomes and/or functional impairments, causing direct and indirect costs of management in the order of billions of dollars annually.^2–6 Most cases can be diagnosed clinically. Newer, noninvasive diagnostic tools, including dermoscopy, high frequency ultrasound, and confocal microscopy, may help in the diagnosis; however, the histopathological evaluation remains the gold standard for diagnosis.^7,8 Current procedural modalities, such as Mohs micrographic surgery, regular excision, cryosurgery, curettage and electrodessication, and radiation therapy, as well as nonsurgical modalities (indicated as monotherapy or as adjuvants), including interferon (IFN), imiquimod, retinoids, and 5-fluorouracil (5-FU), have demonstrated to be effective for the treatment and prevention of NMSC.^5,6,9,10 Our focus is to describe new developments in the prevention and treatment of NMSC. Some considerations are taken in regard to actinic keratoses (AKs), which represent the initial intraepidermal manifestation of keratinocyte abnormal transformation that may potentially progress to SCC.¹¹

Prevention

The approach to NMSC prevention begins with the identification of high-risk individuals. Individuals with UVR-related skin cancers (i.e., BCC and SCC) usually have the following qualities: Fitzpatrick I–II skin phototype; male gender; older age (40–79 years old); history of chronic UVR exposure; living in lower latitudes (closer to the equator); predisposal to genetic disorders, such as xeroderma pigmentosum (XP), basal cell nevus syndrome (BCNS), epidermodysplasia verruciformis, and albinism; immuno-suppression; status post-organ transplantation; exposure to ionizing radiation, coal tars, soot, petroleum oils, polycyclic aromatic hydrocarbons, and arsenic; burn scars; and infection with human papillomavirus types 16, 18, 30, and 33 (SCC).^2,11,12

Primary prevention includes sun-protective behavioral measures, such as avoidance of excessive sun exposure, particularly between 11 a.m. and 2 p.m.; avoidance of artificial UV sources, such as tanning beds and prolonged UV treatments; application every 3 to 4 hours of a broad-spectrum sunscreen with UVB protection of at least 30 sun protection factor (SPF) and high and extended UVA protection; reapplication of sunscreen in cases of excessive sweating or swimming; and the use of protective clothing.^4,6,11–15

Secondary prevention includes a full body examination for early detection and several treatment modalities that may prevent further development and recurrence. Among these treatments, topical and systemic retinoids have demonstrated their efficacy in decreasing the risk of developing BCC and SCC.^5,16–18 Retinoids induce apoptosis, arrest growth, stimulate differentiation of tumor cells during carcinogenesis,^19–21 and downregulate the overexpression of cyclooxygenase-2 (COX-2) induced by UVR, causing a decrease in prostaglandins, which are increased in NMSC.^22–25 Isotretinoin and acitretin are the most common systemic retinoids used for NMSC chemoprevention.^26,27 They may decrease the morbidity and mortality seen in patients with single, high-risk, and multiple primary cancers, particularly in those with organ transplants, immunosuppression, xeroderma pigmentosum, and BCNS.^5,26,28,29 Several studies have demonstrated the efficacy of topical all-trans-retinoic acid (tretinoin) for the treatment of AKs, thus preventing their progression to SCC.^9,29–36

The consumption of a low-fat diet has also been associated with a reduction in the number of AKs in individuals with a history of NMSC^37,38 and in animal models.³⁹ Current evidence does not support the association of fat intake with the development of BCC.³⁹

Newer agents currently in development or being studied for the prevention of NMSC include the following:

Perillyl alcohol (POH)—a hydroxylated monoterpene found in essential oils of plants, including citrus peels, mints, and celery seeds⁴⁰ with antitumor activity in UV-induced skin carcinogenesis⁴¹ inducing apoptosis, and suppression of inflammation, oxidative stress, the activity of ornithine decarboxylase, thymidine incorporation into deoxyribonucleic acid (DNA), the Ras pathway, and alteration of the Bax:Bcl-2 ratio in mice skin.^42–45

T4 endonuclease 5 (T4N5)—an enzyme involved in the reparation of cyclobutane pyrimidine dimers generated in the DNA molecule after exposure to UVR, which have been associated with the generation of NMSC.^46–48 A topical formulation has been developed and T4N5 has been found within the cytoplasm and nucleus of epidermal Langerhans cells and keratinocytes in preliminary studies. Early clinical studies have demonstrated its efficacy (i.e., reduction of 68% of AKs in 30 patients with XP compared with placebo) with good tolerability in the prevention of NMSC.^46,47,49 Two well-controlled studies, a Phase 3 and a Phase 2b, are currently being conducted in XP and in renal transplant recipients, respectively, to further evaluate T4N5's safety and efficacy.^50,51

DL-α-tocopherol—the active component in vitamin E and a well-known antioxidant that, when topically applied in mice, has been found to prevent cancer and decrease UVB-induced cytokine stimulation and inflammation.^52–55 However, DL-α-tocopherol has failed to demonstrate any significant protective effect in well-controlled clinical trials.⁵⁵

α-difluoromethylornithine (DFMO)—an irreversible inhibitor of ornithine decarboxylase, the rate-limiting enzyme in polyamine synthesis.^56,57 Polyamines (e.g., putrescine, spermidine, spermine) bind to DNA, ribonucleic acid (RNA), and phospholipids and control DNA replication, transcription, and translation. They are imperative for normal cellular proliferation and differentiation^58,59 and are up-regulated during the promotion phase of chemically and UVB-induced skin carcinogenesis models.²⁹ DFMO 10% ointment was applied to one forearm each of 48 patients with moderate-to-severe AKs for six months in a randomized, controlled trial (RCT).⁶⁰ A significant reduction in the number of AKs was found in comparison to placebo, with a small percentage of patients reporting moderate-to-severe local adverse events (AEs). In a follow-up study,⁶¹ a significant reduction in the levels of p53 was found, while levels of proliferating cell nuclear antigen (PCNA) and the apoptotic rate were not significantly altered in the DFMO-treated skin. Another RCT⁶² reported significantly less new NMSCs in 291 patients with a history of previous NMSC who received oral DFMO for 4 to 5 years compared with placebo. Current RCTs are comparing the efficacy of DFMO with or without triamcinalone in the prevention of NMSC in patients with AKs⁶³ and the effectiveness of DFMO in preventing skin cancer in patients who have previously received treatment for early-stage skin cancer.⁶⁴

Interferon

Interferon (IFN) works by binding to receptors located on target cells. The exact mechanism of action of these cytokines remains unclear, but interferons are known to have many important effects for the treatment of skin cancer, including antiproliferative effects (i.e., inhibition of mitosis and growth factors, activation of pro-apoptotic genes, and promotion of antiangiogenic activity) and up-regulation of the immune system in the skin.

Intralesional IFN-α2b injections three times per week (3x/wk) for 2 to 3 weeks have shown to be effective for the treatment of AKs compared with placebo.⁶⁵ Limitations of intralesional IFN's clinical use in AKs include multiple injections and many office visits. The topical application of IFN-α2b gel 30 million IU/g four times daily for four weeks obtained clinical, although nonsignificant, improvement of AKs compared to placebo.⁶⁶

Intra- and perilesional IFN represent effective nonsurgical alternatives to treat BBCs, obtaining clearance rates between 70 to 100 percent.^67,68 Its use is limited by its cost, safety profile, and the inconvenience of returning to a physician's office for multiple injections.^69,70 A multicenter RCT where IFN-α2b was used to treat 172 patients with biopsy-proven BCC found the optimal dose to be 1.5 million IU intralesionally administered 3x/wk for three weeks. Significant clinical and histological clearance was obtained when compared with placebo.⁷¹ Similar doses used for recurrent or morpheaform BCCs failed to demonstrate clinical efficacy.⁷² Twenty BCCs received treatment with intralesional IFN-α2b 3x/wk for three weeks at a dose of 1.5 million IU for lesions less than 2cm in diameter and three million IU for lesions 2cm or greater in diameter. More than half of lesions completely responded clinically and histologically at eight weeks of follow up. In those lesions that completely responded, only one recurrence was reported at five years of follow up.⁷³

Intralesional IFN has also shown efficacy for the treatment of SCCs. A total of 33/34 SCCs localized to sun-exposed areas showed complete histological response after treatment with intralesional IFN-α2b at a dose of 1.5 million IU, 3x/wk for 3 weeks.⁷⁴ The same regimen was used in 27 invasive and seven in-situ SCCs, ranging in size from 0.5 to 2.0cm, obtaining clinical and histological clearance rates (CRs) in 97 percent at 18 weeks, with a 96.2 percent CR in the invasive lesions. Almost 94 percent of patients and investigators rated the cosmetic results as “very good” or “excellent.”⁷⁵ In a study evaluating transplant-associated metastatic SCC, a combination therapy with retinoids and IFN-α led to a low number of complete responders.⁷⁶ A combination of IFN-α and IFN-γ applied peri- and intralesionally 3x/wk for three weeks was used for the treatment of 12 BCCs and four SCCs in 16 elderly patients with extensive and recurrent BCCs or SCCs who had previously failed other treatments. Almost half of the patients had complete response to the treatment.⁷⁷

AEs from IFNs are usually dose-dependent and will remit after dose-reduction or cessation of therapy. They include anorexia, influenza-like symptoms, skin necrosis at injection site, rhabdomyolysis, hypotension, arrythmias or tachycardia, neurological effects, gastrointestinal disturbances, erectile dysfunction, fatigue, hepatotoxicity, hematological toxicity (leukopenia, thrombocytopenia, anemia), hypothyroidism, and ocular toxicity. Cases of exacerbation of psoriasis and induction of de novo psoriasis and psoriatic arthritis have been reported.⁷⁸ Interferon use during pregnancy has not formally been studied; however, it has been administered to rhesus monkeys at doses equivalent to those administered in humans and has abortifacient effects.⁷⁹

5-Fluorouracil

Topical 5-FU decreases cell proliferation and induces cellular death, particularly in cells with high mitotic rates, through inhibition of thymidylate synthetase, which interferes with DNA synthesis.^80,81 Evidence suggests that 5-FU used as a topical chemotherapeutic agent in NMSC has been effective for the treatment of superficial BCC (sBCC), in-situ SCC, and AKs. Due to lack of penetration through the dermis, 5-FU is generally not recommended for invasive BCCs and SCCs.^5,81 Several studies have reported the efficacy of topical 5-FU 5% ointment for the treatment of NMSC, with most patients rating their cosmetic results as “good” to “excellent.” However, high recurrence rates have been frequently reported.^82,83,85

A total of 103 patients with invasive BCCs were treated with 5-FU in concentrations from 1 to 5% twice daily for a range of 1 to 5 weeks. Although in 25 percent of the cases the treated area had normal appearing skin, and the remaining ulcerated cases had minimal neoplastic tissue, all cases presented histological evidence of invasion after microscopically controlled excision. 5-FU controlled the superficial part of the BCCs, leading to a relatively benign clinical appearance of the tumors, causing delays in definitive treatment for up to 18 months.⁸⁴ Several studies have compared the efficacy, safety, and cosmetic outcomes of 5-FU and photodynamic therapy (PDT) for the treatment of Bowen's disease (BD) in both immunocompetent⁸⁶ and organ transplant patients.^86–88 In both groups of patients, PDT demonstrated superior efficacy, less recurrences, and less AEs during treatment and after 12 months of follow up.^87,88 In addition, PDT obtained better cosmetic outcomes than 5-FU. Hamouda et al⁸⁸ treated 10 patients with multiple unresectable SCCs in XP patients with 5-FU twice daily for an average of six months. Although clinical superficial regression was reported in most of the patients, post-treatment biopsies in five patients showed clearance of the epidermal ulceration in one patient and the persistence of deep dermal components in the remaining four samples.

A total of 14 elderly patients with biopsy-proven aggressive, multiple or recurrent SCCs that had failed prior standard treatments (including topical 5-FU) were treated with oral 5-FU 175mg/m² daily for three weeks every five weeks.⁸⁹ Oral 5-FU was effective and well tolerated, although larger studies are needed to evaluate its real clinical value.

In regard to the topical treatment of AKs, 5-FU has demonstrated high efficacy rates^90–94; however, recurrences are frequently reported,⁹⁵ and low adherence due to AEs has been related to 60 percent of treatment failures.⁹⁶ Topical 5-FU has the advantage of also treating clinically undetectable (subclinical) AKs.⁹³ The combination of 5-FU and imiquimod has resulted in faster responses and fewer AEs.⁹⁷ Current studies are evaluating the role of 5-FU in the prevention of new NMSC on the face and ears.⁹⁸

Imiquimod

Imiquimod stimulates the innate- and cell-mediated immune responses, potentiating their antiviral, antitumoral, and immunoregulatory properties. Imiquimod 5% cream is approved by the United States Food and Drug Administration (FDA) for the treatment of external genital and perianal warts; nonhyperkeratotic, nonhypertrophic AKs; and sBCC mostly in patients in whom surgery is not an option.⁹⁹ A new concentration of imiquimod (i.e., 3.75%) has been approved by the FDA for the treatment of AKs on the face or balding scalp and will be available soon in US and Canadian markets. In addition, generic imiquimod 5% cream is currently available.

The antitumoral mechanisms of action of imiquimod include the following:

1. Induction of the production of cytokines, including IFN-α, tumor necrosis factor (TNF)-α, interleukin (IL)-1, IL-2, IL-6, IL-8, and IL-12, by human peripheral blood mononuclear cells

2. Stimulation of monocytes, macrophages, and toll-like receptor (TLR) 7 and 8-bearing plasmacytoid dendritic cells (DCs) and epidermal Langerhans cells. The stimulation of TLRs induces the production of pro-inflammatory cytokines involved in the modulation of the innate immune system¹⁰⁰

3. Stimulation of the production of IL-6, IL-8, and IFN-α, by keratinocytes resulting in a Th1-dominant response^100–106

4. Increase in the levels of type I IFN, which improves the response to endogenous IFN-α, usually low in AKs. Suppression of type I IFN signaling proteins is an early event leading to SCC¹⁰⁷

5. Induction of FasR (CD95), a member of the tumor necrosis receptor family, involved in apoptotic pathways

6. Induction of pro-apoptotic pathways associated with the B cell lymphoma/leukemia-2 (Bcl-2)-associated X (Bax) protein^108,109

7. Induction of caspases 9 and 3, which have been linked with stress signaling, mitochondrial death pathways, and apoptosis¹¹⁰

8. Induction of E-selectin on blood vessels of invasive SCCs, which is a ligand for lymphocyte antigen expressed by skin resident T-cells that are in charge of immunosurveillance, and is usually absent in SCCs⁹

9. Reduction of T-regulatory cells (expressing the transcription factor FOXP3) infiltrating SCCs. These FOXP3+ T-regulatory cells cause suppression and impairment of effector T-cells (responsible for immunosurveillance), surround the tumor, and prevent T-effector cells from reaching the tumor⁹

Imiquimod, as topical therapy for AKs, has the advantage of treating multiple lesions including subclinical AKs. CRs of AKs treated with imiquimod range from 45 to 85 percent,^{108,111–116} with recurrence rates up to 16 percent after 18 months of treatment and approximately 20 percent at 24 months follow up.^111,117

Actinic keratosis. A total of 829 patients with a total of 7,427 AKs were treated with imiquimod 5% cream 3x/wk for four weeks followed by a four-week rest. A second four-week treatment period was applied in cases of remaining lesions. Imiquimod obtained high clearance rates and reduced 85 percent of the total number of lesions.¹¹⁸

In several multicenter, prospective, double-blind RCTs, imiquimod 5% cream was evaluated in a total of 1,493 patients with multiple AKs on the face or balding scalp. Imiquimod was applied daily, 2 to 3x/wk for a maximum of 24 weeks.^{115,116,119–121} Overall, 3x/wk regimens result in a higher clinical response than 2x/wk. The median percentage reduction in the number of AKs from baseline was greater than 86 percent with 3x/wk regimens and 83.3 percent with 2x/wk applications.¹²⁰ Most common AEs included local skin reactions, such as erythema, scabbing/crusting, flaking/ scaling/dryness.

A RCT was conducted applying imiquimod or vehicle twice weekly for eight weeks following 3- to 5-second cryotherapy of target AKs within a 50cm² field on the face or scalp. At 12 weeks, the CR was similar for imiquimod and vehicle, but fewer total AKs were noted with imiquimod. A progressive reduction in subclinical AKs with imiquimod compared with a progressive increase with vehicle was noted. In addition, more subjects treated with imiquimod achieved clearance of subclinical AKs than with the vehicle. Imiquimod post-cryotherapy may increase clearance of subclinical and total AKs.¹²²

In a multicenter RCT,¹²³ 43 post-transplant patients receiving immunosuppressive therapy within the prior six months concomitantly presenting with AKs were treated with imiquimod or vehicle 3x/wk for 16 weeks. Imiquimod proved to be effective and well-tolerated for the treatment of AKs in immunocompromised patients.

Two concentrations (3.75% and 2.5%) of imiquimod cream have been studied in order to optimize its topical use. Imiquimod 3.75% applied daily for two-week treatment cycles with two weeks without medication in between cycles, for four months, maintained the efficacy achieved by 5% imiquimod and was well-tolerated, when measured by median percent reductions in the number of AKs.¹²⁴ Currently, clinical studies are assessing AK treatments using imiquimod 3.75% cycle dosing as field- and lesion-targeted treatment.¹²⁴ Furthermore, in four Phase 3 studies, treatment of 5 to 20 AKs on the entire face or balding scalp was evaluated using imiquimod 2.5% and 3.75% creams applied daily in two cycles of either 2 or 3 weeks, separated by a matching no-treatment interval (2/2/2 or 3/3/3). More than 40 percent of the patients who achieved a complete response at eight weeks post-treatment with the 3.75% cream maintained a recurrence-free status after 12 months of follow up. Complete responders who had recurrences at the follow-up visit had less lesions compared with patients treated with placebo.¹²⁵

Superficial basal cell carcinoma (sBCC). As a noninvasive treatment modality for sBCC, imiquimod may have some advantages over surgical procedures, especially in regard to cosmetic outcomes.¹⁴⁷ RCTs have been conducted using imiquimod 5% cream for histologically confirmed nodular BCC (nBCC) and sBCC. Various treatment regimens were applied in these studies (for example: twice-daily, once-daily, 5x/wk, 3x/wk), achieving responses up to 100 percent. Regimens with more frequent imiquimod applications (once daily and twice daily) were associated with the best responses. Once-daily or 5x/wk application groups had the highest efficacy rates with acceptable safety profiles.^148,149 Most common AEs included local skin reactions.

In RCTs evaluating a total of 890 patients with histologically proven sBCC who received treatment with imiquimod 5% cream 5x/wk or 7x/wk for six weeks or placebo,^145,150 it was shown that imiquimod-treated groups obtained significantly better clinical and histological responses. Local skin reactions were reported more often in the 7x/wk than in the 5x/wk imiquimod-treated group.

Nodular basal cell carcinoma (nBCC). In two Phase 2 studies, the highest response rate achieved was 76 percent using imiquimod 5% cream twice daily for 12 weeks.¹⁵¹ The responses in these studies were not as high as those seen in studies where sBCC was treated with imiquimod 5% cream. This type of treatment modality applies better for those patients in which surgery, radiotherapy, or cryotherapy is not an option. In a RCT,¹⁵² 93 patients with biopsy-proven nBCC applied imiquimod 5% cream in several different treatment regimens for six weeks. At the end of treatment, more frequent applications (i.e., 3x/wk) under occlusion achieved better results than less frequent applications (i.e., 2x/wk); however, no significant differences were found between the groups. An evidence-based systematic review supports the use of topical imiquimod as monotherapy for sBCC and SCC in situ. The use of this modality may be limited to patients with small tumors in low-risk locations who cannot undergo surgery or other treatment regimens for which long-term clearance rates have been determined.¹⁵³

Resiquimod

Resiquimod is a TLR 7/8 agonist that has similar effects as imiquimod on monocytic cells, although it is 10 to 100 times more potent than imiquimod.^154–156 In addition, it induces IL-1 receptor antagonist (IL-1ra), granulocyte-colony stimulating factor (G-CSF), granulocyte macrophage-colony stimulating factor (GM-CSF), macrophage inflammatory protein (MIP)-1α, MIP-1β, and monocyte chemotactic protein (MCP-1).¹⁵⁶ A total of 132 patients with 4 to 8 AKs on the face or the balding scalp applied resiquimod at different concentrations once daily, 3x/wk for four weeks obtaining complete responses ranging from 40.0 to 74.2 percent. No statistical difference was found among the various concentrations of resiquimod, but better tolerance was seen with lower concentrations.¹⁵⁷

Ingenol Mebutate

Ingenol mebutate (IM) is an extract from the plant Euphorbia peplus, also known as milkweed, which has been used for several years as a home remedy for skin conditions, such as AKs and skin cancers.^158–161 It has a dual mechanism of action first causing chemoablation and disruption of the plasma membrane with loss of the mitochondrial membrane potential and mitochondrial swelling, specifically targeting dysplastic keratinocytes, leading to fast cell death by primary necrosis. It also induces rapid healing and restoration of the normal clinical and histological morphology.¹⁵⁸ This necrosis allows IM to remain active in the presence of apoptosis-resistant tumor cells.^162,163 Second, IM induces neutrophil infiltration and the production of tumor-specific antibodies and pro-inflammatory cytokines, leading to an antibody-dependent cellular cytotoxicity that eliminates residual cells.¹⁶⁴ IM has been evaluated in several controlled studies for the treatment of AKs. A total of 280 patients with AKs have been treated with topical IM gel at concentrations of 0.025% and 0.05% achieving significantly better clinical results than the vehicle.^159,165 Common AEs included erythema, flaking/scaling, and crusting. There were no reports of scarring. Current Phase 3 studies are evaluating IM topical gel at 0.015% for the treatment of AKs located on the face and scalp^166,167 and 0.05% for non-head locations.^168,169

Cyclooxygenase-2 inhibitors

Cyclooxygenase (COX) is the rate-limiting enzyme in the pathway of synthesis of prostaglandins from arachidonic acid. Increased production of prostaglandins may be associated with the development of UV-induced NMSC.¹⁷⁰ Prostaglandin E2 (PGE2) has also been implicated in the induction of bcl-2 gene expression, IL-6, and the inhibition of apoptotic pathways. It has been determined that of the two COX isoforms, COX-2 is overexpressed in several human neoplasias, including colon carcinoma, SCC of the esophagus, and skin cancers (AKs, melanoma, and NMSC), and in metastasic murine mammary tumor cells.^171,172 UVB is a well-known factor capable of inducing COX-2 in humans and in animal models. In the latter, a decrease in the number of skin tumors has been obtained after the administration of a selective COX-2 inhibitor.^22,23,173 Several clinical trials have evaluated the efficacy of selective COX-2 inhibitors for the chemoprevention of AKs and NMSC.^174–179 Topical diclofenac 3.0% gel in hyaluronan 2.5% gel, which reduces diclofenac's epidermal diffusion, has shown to be safe and effective in the treatment of AKs. A combination of diclofenac gel and cryosurgery demonstrated to be superior to cryosurgery alone in a multicenter RCT.¹⁸⁰ As monotherapy, diclofenac/hyaluronan topical gel was applied twice daily to 195 patients with AKs in a RCT, achieving significantly better clearance scores than placebo after 60 days of treatment.¹⁸¹ A recent study showed that short-term non- steroidal anti-inflammatory drug (NSAID) use (<5 years), but not long-term use, had significant protective effect for SCC and BCC compared with non-NSAID users. Non-aspirin NSAID use showed significant protective effects against BCC. This effect was not seen with aspirin.¹⁸² In a In a small series, diclofenac has been shown to be clinically and histologically effective and well-tolerated for the treatment of Bowen's disease.^183–185 Animal studies have shown that other selective COX-2 inhibitors, such as celecoxib, and nonselective COX inhibitors, such as indomethacin, are also effective for the chemoprevention of skin cancers.^22,25,186 Currently, clinical trials are evaluating the use of celecoxib for the prevention of AKs¹⁸⁷ and for the prevention of BCCs in patients with BCNS.¹⁸⁸

Betulinic Acid

Betulin, betulinic acid, oleanolic acid, lupeol, and erythrodiol are pentacyclic triterpenes that are contained in the outer bark of birch (Betula alba cortex) and are found to have antiviral, antimicrobial, hepatoprotective, and antitumoral effects.^189,190 Importantly, they promote keratinocyte differentiation and induction of cytotoxic, antiproliferative, and apoptotic effects on tumor cells.¹⁹¹ In a randomized trial, Huyke et al¹⁹¹ treated 45 patients (with <10 AKs each) with either a topical betulin-based oleogel twice daily, croyotherapy, or a combination of the two. Treatment with betulin-based oleogel was well-tolerated and showed efficacy in treating AKs. Further controlled studies on larger sample sizes investigating the use of betulinic acid for the treatment of NMSC are warranted.

Cyclopamine

Cyclopamine is a steroidal alkaloid extracted from the corn lily Veratrum californiucum, which is very common in subalpine meadows.¹⁹² It has shown inhibitory activity against the hedgehog family of intercellular signaling proteins. This membrane complex, particularly the Sonic (hedgehog) (SHH) subtype, present in mammals, has recently been implicated in the development of BCC in the white population.^193–195 The hedgehog signaling pathway is modulated by two transmembrane proteins, the tumor suppressor patched (PTCH), and the proto-oncogene smoothened (SMO). The signaling is activated when the SHH protein binds directly to PTCH receptor (encoded from the PTCH-1 gene), causing its inhibition and further repression of the expression of the target genes.^193,195 When the SHH protein is not present, PTCH-1 inhibits the activity of smoothened (SMO), which is a key activator of the pathway, thus preventing the expression of target genes, such as glioma transcription factor-1 (GLI-1).^195,196 Binding of SHH to the PTCH receptor eliminates that inhibition of PTCH on SMO, resulting in a dysregulated hyperactivity of the hedgehog pathway.¹⁹³ Good examples of the activation and dysregulation of the hedgehog pathway represent human sporadic development of BCCs, and BCNS, where germ-line mutations in the PTCH-1 gene or in any of the components of the hedgehog pathway contribute with their development.^196–198 Other neoplasias that have been related to mutations in the hedgehog signaling pathway include breast carcinomas, odontogenic keratocysts, medulloblastomas, sebaceous nevus, trichoepitheliomas, fibrosarcomas, and dermatofibromas.^193,195 Clinical studies are needed to evaluate the efficacy and safety of topical cyclopamine for the treatment of NMSC.

GDC-0449

GDC-0449 is another hedgehog pathway inhibitor specifically targeting the SMO membrane protein. It is more potent and has more favorable properties than cyclopamine.^196,199 Studies in animal models of medulloblastoma and xenograft models of human colorectal cancer and pancreatic carcinoma have demonstrated GDC-0449's antitumoral activity.^199,200 A case of a 26-year-old patient with the diagnosis of cerebellar medulloblastoma resistant to multiple therapies with further systemic metastatic disease received treatment with a daily oral dose of 540mg of GDC-0449 for several months.²⁰¹ Although the response (tumor regression) was impressive and fast, it was transient and incomplete, and the patient died afterwards. A Phase 1, multicenter, open-label study was conducted to evaluate safety and tolerability (Stage 1) and pharmacokinetics (Stage 2) of GDC-0449 in patients with metastatic or locally advanced BCC who have failed standard therapy.¹⁹⁹ A total of 33 patients were enrolled: 18 with metastatic tumors and 15 with locally advanced BCC. GDC-0449 had antitumor activity in those patients with advanced BCC. The most frequent AEs included fatigue, muscle spasm, weight loss, and hyponatremia. Current clinical Phase 2 studies are being conducted to evaluate safety and efficacy of GDC-0449 for the treatment of BCNS and sporadic BCCs.^202,203

Photodynamic Therapy

Photodynamic therapy (PDT) represents a noninvasive option for the treatment of numerous, thin, nonhyperkeratotic AKs.^204,205 Aminolevulinic acid (ALA) or methyl-aminolevulinate (MAL) is first applied to the skin at the target area, where it is absorbed and accumulates inside dysplastic and neoplastic cells up to 10 times faster than in normal cells.^206–209 It is converted to a potent photosensitizer, protoporphyrin IX, which, when exposed to a light source, activates and leads to the formation of reactive oxygen species and destruction of dysplastic cells.¹¹ When undergoing PDT, patients often complain of stinging, burning, and itching at the site of treatment. Erythema, scaling, and crusting may be evident after treatment, but usually the area heals with no evidence of scarring.⁵

Field-directed therapy for the prevention of new cancer formation may be beneficial to individuals with multiple precancerous lesions. In a study by Apalla et al,²¹⁰ 45 patients previously diagnosed with NMSC, including BCCs and SCCs, on the face or scalp, with AKs located symmetrically in the same region, were treated with PDT in order to evaluate its effect on the formation of new NMSCs. Two treatment cycles of ALA-PDT, one week apart, were used on one side of the face, while placebo-PDT was applied similarly to the other side of the face. It was found that the ALA-PDT-treated areas were significantly delayed in the mean time until appearance of new lesions and had a decreased number of new lesions at three months and at 12 months of follow up compared to placebo, demonstrating the potential use of PDT for NMSC prophylaxis.

In an investigator-blind RCT,²⁰⁹ 243 patients with a total of 1,403 AKs on the face and scalp received either ALA or vehicle followed by 14 to 18 hours of incubation and exposure to blue light. A significant difference in clearance rate was seen when compared to placebo at 12 weeks of follow up. A double-blind, multicenter RCT211 evaluated the use of two treatments of MAL, red light PDT, one week apart, in 80 patients with AKs on the face or scalp. Three months post-treatment, this regimen was significantly better than placebo in clearing lesions. In a multicenter, randomized, open study, 211 patients with 413 total AKs were treated with different MAL-PDT regimens, and a single treatment of MAL-PDT was found to be as effective as two treatment sessions, one week apart for thin AKs; two treatment sessions are recommended for thicker or nonresponsive lesions.²¹²

When comparing PDT to cryotherapy, results have varied, but cosmesis is found to be superior with PDT.^206,213 When compared to 5-FU for the treatment of AKs on the face or scalp of 36 patients,²¹⁴ equivalent efficacy was found; however, ALA-PDT was better tolerated than 5-FU. In 30 patients with a total of 256 AKs on the dorsal hands or forearms, two sessions of ALA-PDT, 15 days apart, were compared to imiquimod 5% cream daily, 3x/wk for four weeks, with a four-week break and another cycle if CR was not achieved. The CR rates were significantly higher for the imiquimod-treated group at one month. At six months, the CR rates were also higher, but did not reach significance. A larger number of patients preferred PDT in regard to procedure and in terms of efficacy.²¹⁵ The efficacy of two sessions of ALA-PDT one month apart followed by either imiquimod 5% cream or vehicle applied daily, 2x/wk for 16 weeks, starting at Month 2 were evaluated in a double-blind, split-face RCT.²¹⁶ Median lesion reduction at one year was significantly greater on the imiquimod-treated side of the face than on the vehicle-treated side.

PDT also represents a treatment option for superficial NMSCs. The depth of penetration makes it less successful for lesions such as nBCCs, although it still remains an option. Past studies have shown that fractionated PDT leads to higher responses than continuous illumination PDT.^8,9 When ALA-PDT with a fractionated illumination scheme was used to treat 552 lesions (430 sBCCs, 20 nBCCs, 32 BDs, 70 AKs), a CR was seen in 95 percent of lesions at an average follow up of two years with a “good” to “excellent” cosmetic outcome in 95 percent of lesions. For lesions larger than 2cm, the CR rate at two years was 89 percent (n=57).²¹⁷

One disadvantage of PDT is the need for multiple office visits to receive the treatment. In an open pilot study, 12 patients with BD and sBCC <2cm in diameter were given two PDT treatments, one month apart, using a lightweight, organic, light-emitting diode (OLED) as a light source suitable for ambulatory PDT. At one year follow up, seven patients remained clear of lesions and all subjects scored the pain level as less than 2, using a numerical rating score of 1 to 10. Routine PDT light sources scored a 6 on the numerical rating scale. Ambulatory PDT may represent a less painful and more convenient method of treatment for NMSC patients.²¹⁸

Currently, there is a Phase 2 trial underway to determine the safety and efficacy of ALA-PDT versus vehicle-PDT for the treatment of AKs and reduction of new NMSC of the scalp or both forearms in solid organ transplant recepient subjects on chronic immunosuppressive therapy.²¹⁹

Lasers

Lasers induce coagulative necrosis, ablation, and hyperthermia, leading to tumor destruction. Many studies have provided evidence that lasers represent a new, effective treatment option for the management of NMSCs.^220–224 A single pass with a CO₂ laser may be used to destroy lesions in the superficial epidermis that are at risk of becoming AKs or SCCs and possibly BCCs.²²⁵

CO₂ laser wavelengths are absorbed by water in the epidermis, resulting in nonspecific tissue destruction. CO₂ lasers have been used to treat sBCCs and Bowen's disease. The use of superpulsed CO₂ lasers was studied in 44 patients with Bowen's disease. The total response rate was 97.7 percent, and clearance after one treatment was achieved in 86.3 percent of patients. A total of five patients achieved clearance after more than one treatment and one patient did not respond. Three patients showed evidence of recurrence at six months, four months, and at one year and five months, with an unknown evolution time. One patient had a serious AE, the formation of a keloid, after treatment. Thirteen other patients experienced minimal local AEs, such as erythema and hypo/hyperpigmentation.²²⁶

In a study by Moskalik et al,²²⁷ the use of neodymium (Nd) lasers for the treatment of facial skin cancers was assessed. In 3,461 patients with 3,534 BCCs and 90 SCCs, pulsed Nd and neodymium-doped yttrium aluminium garnet (Nd:YAG) lasers were used, and patients were followed for three months to five years. Recurrences were seen in 1.8 percent of patients with BCCs treated with pulsed Nd laser, in 2.5 percent of patients with BCCs treated with Nd:YAG laser, and in 4.4 percent of patients with SCCs treated with pulsed Nd laser. Irradiation using Nd lasers represents a useful way to treat facial skin cancers.²²⁷

Afamelanotide (CUV1647)

UVB induces the synthesis of melanin in melanocytes through the release of a-melanocyte stimulating hormone (a-MSH) from cutaneous melanocytes and keratinocytes. This effect is produced after UVB has already caused substantial damage to keratinocytes.²²⁸ The increase in melanin may have protective effects against UVB. Afamelanotide (CUV1647) is an analogue of a-MSH with higher potency and longer action than the naturally occurring hormone.²²⁹ It is currently being studied to evaluate its efficacy in reducing the number of AKs and SCCs in organ transplant recipients on chronic immunosuppressive therapy.²³⁰

Epidermal Growth Factor Receptor Inhibitors

The epidermal growth factor receptor (EGFR) gene codes for a transmembrane tyrosine kinase receptor related to cellular proliferation and epithelial development.²³¹ Its overexpression has been associated with non-small-cell-lung cancer, colorectal cancer, SCC of the head and neck, and SCC.^231–233 Several antitumoral agents that block this receptor that have been developed for the treatment of those cancers and are currently in different phases of clinical evaluation for the treatment of NMSC, include gefitinib²³⁴ and erlotinib.^235–238 In addition, anti-EGFR monoclonal antibodies have been developed and are currently in clinical trials, including cetuximab²³⁹ and the fully human panitumumab.^240–244

Capecitabine

Capecitabine is an oral prodrug of 5-FU that has demonstrated efficacy for the treatment of SCC, particularly in combination with IFN. As a monotherapy, it was administered orally at a dose of 1g/m² divided into two daily doses on Days 1 through 14 of a 21-day treatment cycle in 14 organ transplant recipients with recurrent SCCs and BCCs. Capecitabine significantly decreased the SCC recurrence rates in these patients.²⁴⁵ Further studies are needed to determine its role in the treatment of NMSC.

Conclusion

The incidence and prevalence of NMSC will continue to rise due to several factors including, but not limited to, the increase in human lifespan, early detection, and awareness campaigns. Preventive measures include sun protection and consumption of low fat diets, along with retinoids and newer agents, such as perillyl alcohol, T4N5, DFMO, and DL-a tocopherol. Although surgical modalities are considered the standard of care for the treatment of NMSC, newer nonsurgical agents targeting key cellular receptors or immunological responses have considerably reduced morbidity and mortality and increased the quality of life of patients. Some of these agents, including imiquimod, IFN, and 5-FU, have consistently demonstrated substantial efficacy when used as monotherapies and in combination with surgical modalities. PDT and lasers have also shown to be effective alone or in combination with topical immunomodulators for the treatment of NMSC and premalignant lesions. Clinical trials evaluating newer treatment options, such as IM, COX-2 inhibitors, betulinic acid, cyclopamine, GDC-0449, resiquimod, afamelanotide, EGFR inhibitors, and capecitabine are obtaining promising results.