Currently available agents are 5-FU, imiquimod, diclofenac, PDT and ingenol mebutate.
5-Fluorouracil Cream: Anti-neoplastic Agent
Topical 5-fluorouracil (5-FU) has had a place in the treatment of AK for many decades [
58]. It is a pyrimidine analog, which disrupts DNA formation by stopping the conversion of deoxyuradilic acid to thymidylic acid [
59]. This prevents cell proliferation preferentially in rapidly dividing cells, especially those of AKs and basal layers of the epithelium. 5-FU is available as a cream in 5, 1 and 0.5% concentrations, and as a solution in 5% and 2% concentrations. It is the most established field treatment for AK, and is considered by some the traditional gold standard to which all other topical agents are compared [
1]. The typical treatment regime is either 5% cream twice daily or 0.5–1% cream daily for 2–4 weeks. The widespread application has the advantage of treating clinically undetectable AKs.
5-FU causes inflammation, erosion, and ulceration during treatment, which is necessary for therapeutic success. For a standard 3- to 4-week period of twice daily application, these side effects arise after the first week and subside approximately 2 weeks after application has ceased, when re-epithelialization has occurred. These transient side effects can result in non-compliance.
0.5% 5-FU cream was developed in an attempt to reduce the inflammation associated with application. It is applied once daily for a month. In 2010, Kaur et al. [
60] analyzed clinical trials from 1965 to 2009 to compare the efficacy of 5% and 0.5% 5-FU cream in treating multiple AKs of the face and scalp. They found that after 4 weeks of treatment, complete clearance for 0.5% 5-FU ranged from 16.7% to 57.8%, and for 5% 5-FU clearance ranged from 43% to 100%. The 5% 5-FU cream had a higher rate of adverse events. It was also stated that there is lack of high-powered clinical trials comparing both groups. However, it appears that 5% 5-FU gives the greater chance of complete clearance despite side effects and should therefore be the preferred treatment.
In 2012, Rhavar et al. [
61] performed a systematic review of 103 studies into the efficacy of 0.5% 5-FU. Only four studies were found to be suitable randomized vehicle-controlled trials. Of 668 patients, the percentage achieving complete clearance of their AKs in the 5-FU group was 19, 28.2 and 52.6% in the 1-, 2- and 4-week-treatment groups. Only 0.85% of those in the vehicle-treated group reached complete clearance. The mean lesion count reduction was 90.2% and 28.3% in the 5-FU and vehicle groups, respectively. It was found by Yentzer et al. [
62] that 0.5% 5-FU cream had a high adherence rate of 86% to the once daily application over a 4-week period. After 4 weeks of twice daily application of 5% 5-FU, sustained complete field clearance at 12 months was seen in 33% of patients [
63].
A number of different methods of reducing side effects have been tried. The application of topical corticosteroids 15 min after 5-FU cream can be helpful to reduce the inflammatory response, and intermittent use of 5-FU cream can cause reduced side effects.
Intermittent, or pulsed 5-FU application is not as widely used currently as previously, but has been shown to reduce side effects, whilst maintaining efficacy for the treatment of AK [
64,
65]. Labandeira et al. [
64] studied the effects of four applications per week for the first week, followed by two applications per week if irritation was intolerable. One disadvantage to this regime is that the time to complete healing of lesions is prolonged with reduced frequency of application, but remains moderately effective in treating AKs.
The use of corticosteroid creams and pulsed therapy may reduce the efficacy of 5-FU treatment [
66]. Side effects can also be minimized in practice by choosing to treat smaller areas only, or visible lesions only after the first week. Exposure to sunlight should be minimal as the treated area is photosensitive and exposure can cause pain.
Imiquimod: Immunomodulator Agent
Imiquimod is an imidazoquinolone immune response modifier that acts on both innate and acquired immunity. It is a toll-like receptor-7 agonist that modifies the immune response in the skin and stimulates apoptosis, thereby disrupting tumor proliferation. Additionally, it induces E-selectin on tumor vessels and consequent infiltration by cutaneous lymphocyte-associated antigen-positive skin-homing cluster of differentiation (CD8+) cytotoxic T cells, and results in histological evidence of tumor regression [
67] and a reduction in tumor cell numbers [
68]. It triggers a range of proinflammatory cytokines including interferon alpha, tumor necrosis factor alpha (TNF-α), and interleukin-12 (IL-12) [
69].
It was first approved as a treatment for AKs in 2004 as a 3 times per week application, on the bald scalp and face, for up to 16 weeks. This has been more recently modified to a twice weekly application, in 4-week application cycles, with a 4-week rest period between these cycles to minimize treatment times and adverse events. A 3.75% cream was first approved in 2010 for once-nightly application for 2 weeks, followed by a 2-week rest period without application, which is then followed by another 2-week treatment period [
70].
Available preparations are 5% and 3.75%. The 3.75% preparation allows for a shorter duration of treatment over a larger skin surface area (200 cm
2 vs. 25 cm
2 for 5% cream) [
71]. Absolute clearance rate is higher for the 5% cream, at 45%, compared to 35% for daily use of the 3.75% preparation [
70], and the median reduction in AK is 83% [
72]. Use of the 5% cream can give up to 57% clearance if used three times per week for 16 weeks [
73‐
75].
Cycle therapy is aimed at reducing local skin reactions. In some studies, 5% Imiquimod was applied 2–3 times weekly for 3–4 weeks, and then reviewed after 4 weeks, with repeated treatment if there were any residual lesions [
76,
77]. Complete clearance was achieved in up to 82% of treatment areas. Imiquimod has also been used in immunocompromised patients with good results, and is generally regarded as safe to use in this sub-group [
78].
In 2010, Hanke et al. [
79] performed two placebo-controlled studies of daily application of imiquimod 2.5% and 3.75% for the treatment of AKs, for two 3-week cycles. This looked at the balding scalp and face only. Up to two packets of 250 mg each, were applied per dose once daily, for two 3-week treatment cycles, with a 3-week no-treatment interval. Efficacy was assessed at 8 weeks post-treatment and found that clearance rates for both treatment options were superior to placebo and had an acceptable safety profile. Complete clearance in the 2.5% cream group was 25%, and in the 3.75% cream group was 34%. Partial clearance rates were higher.
A similar study by Swanson et al. [
80] was done to compare efficacy of 2.5% and 3.75% cream in two 2-week cycles. Complete clearance rate with daily application was 30.6% for 2.5% cream and 35.6% for 3.75%, which would suggest that a shorter interval between applications improves outcome. Side effects include skin irritation and erythema, with rarer consequences being flu-like symptoms and lymphadenopathy [
81].
In a randomized trial comparing imiquimod against cryotherapy in the treatment of AK over a 12-month period, it was found that repeated cryotherapy (up to 4 sessions) achieved a higher complete lesion clearance rate (85% vs. 66.9%), while cosmetic outcome was better with imiquimod (likely related to lower incidence of hypopigmentation) [
82]. In another split-face study comparing imiquimod against PDT, the authors found that there was no significant difference in 100% or 75% response to either treatment regimen, but mean lesion reduction rate was superior in the PDT group [
83].
Tolerability of 5% imiquimod was compared to that of MAL–PDT in a randomized controlled trial (
n = 58) [
84]. The patients were asked for example, about pain, side effects and overall satisfaction. Overall, the two treatments were approximately equitable with the PDT group scoring themselves as ‘very satisfied’ slightly more frequently.
Recommended therapy for the face and scalp, or large areas up to 200 cm
2, could therefore usefully be suggested as a short course of 3.75% imiquimod cream daily for 2-week cycles, twice, to give 35.6% of patients complete clearance. Licensed use of 5% cream is for small skin areas up to 25 cm
2, 2–3 times per week for up to 4 months, although 1 month is usually sufficient [
85]. Evidence suggests 12- to 16-week treatment will give complete clearance in 50% of patients [
86]. If inflammation is intolerable, frequency of application can be reduced to once or twice per week with preservation of efficacy. The addition of lesion-directed cryosurgery prior to application gives greater clearance than either therapy alone [
79].
There is evidence to suggest that patients may develop T cell memory after treatment with imiquimod, which is likely to reduce the risk of developing further AKs. The 5% preparation has also been shown to be effective and safe in solid organ transplant patients covering up to 100 cm
2 skin surface area [
78,
87].
A sister drug called resiquimod, which is currently an investigational product, is 10–100 times more potent than imiquimod. A European phase II study of daily applications 3 times per week for 4 weeks found that clearance rates ranged from 40 to 74.2%. The lower concentrations were better tolerated and as effective as the higher concentrations [
88].
Diclofenac Sodium 3% Gel: Anti-inflammatory Agent
Diclofenac 3% gel is a nonsteroidal anti-inflammatory formulated with 2.5% hyaluronic acid. It is a popular treatment in Germany, but not widely used in Australia. Diclofenac is a cyclooxygenase (COX) inhibitor exerting its antitumor effects via inhibition of the COX-2 pathway and by inhibiting up regulation of the arachidonic acid cascade. The production of prostaglandins from arachidonic acid may play a role in UV-B-induced skin cancer [basal cell carcinoma (BCC) and SCC] [
89], and diclofenac’s inhibition of this cascade may explain its efficacy in AK treatment. This effect may be mediated via inhibition of angiogenesis and induction of apoptosis. There is evidence that diclofenac induces regression of AKs [
90,
91].
A 2005 meta-analysis of three randomized trials (
n = 364) found that treatment of AKs with diclofenac gel led to complete resolution in approximately 40% of patients, whereas there was only a 12% rate of complete resolution of those treated with placebo [
92]. Diclofenac resulted in 100% AK resolution in 50% of the subjects after 3 months of twice daily application. This extended period of treatment reduces compliance, but a shorter duration is reportedly less efficacious [
93]. Diclofenac treatment is well tolerated with minimal irritation and inflammation. Adverse reactions can include itch, xerosis, and contact dermatitis [
94].
Recommended dosing is twice per day for 90 days. When it is used after cryotherapy, diclofenac has been shown to give greater complete lesion clearance compared to cryosurgery alone (64% vs. 32%, respectively) [
95]. Tolerance is better than the twice daily application of 5-FU cream, but appears to be slightly less effective when used alone. Long-term data are restricted to a single uncontrolled study [
96] which shows similar efficacy at 12 months to ingenol mebutate (18% complete clearance compared with 19.5%, respectively).
Photodynamic Therapy (PDT): Photosensitizing Agent
Photodynamic therapy is a two-step procedural field therapy, beginning with the topical application of a photosensitizing agent to the treatment area.
Photosensitizing agents are 5-ALA prescribed in the form of an 8 mg adhesive patch, under red light (630 nm); a 78 mg/g nanoemulsion gel, under red light (630 nm) or a 20% solution under blue light (417 nm) and MAL in the form of a 16.8% cream, under red light (630 nm). Licensing of these topical products varies between countries, for example MAL is widely available worldwide, but is the only preparation available in Australia. The 5-ALA 20% solution is only licensed in the US, Korea, Brazil, Mexico, Argentina, Chile and Columbia. These prodrugs are converted by the heme biosynthetic pathway to protoporphyrin IX (PpIX).
After an incubation period whereby the prodrug accumulates preferentially in dysplastic actinic keratosis cells, the area is then illuminated by an appropriate wavelength light. This causes the activation of PpIX and produces reactive oxygen species. The ROSs produced upon light exposure cause apoptosis and necrosis of target tissue, resulting in cell death [
97].
Treatment is currently aimed at patients who have had difficulty adhering to topical field therapies, AK lesions resistant to topical therapies or those with concerns regarding the cosmetic results of treatment [
2]. A typical regime for nonhyperkeratotic, non-pigmented AKs would be the direct application of 20% ALA topical solution to lesions on the face or the scalp for 18–24 h, followed by blue light for 16 min and 40 s; or 16.8% MAL application for 3 h followed by red light for 7–10 min [
98]. A follow-up treatment is recommended for lesions that have not completely resolved after 8 weeks. A single treatment is used for thin or moderately thick lesions, repeated after 3 months if not clinically cleared [
99,
100].
PDT is usually well tolerated and clearance rates of up to 90% after two applications have been reported [
101]. MAL is used most commonly internationally as it selects and penetrates dysplastic cells better than other commercially available products [
102]. PDT with MAL achieved 100% resolution in up to 82% of subjects and cleared 90% of total AK lesions in a 2008 randomized, double-blind, placebo-controlled study by Pariser et al. [
103].
In a recent study evaluating the effect that ablative fractional laser resurfacing (AFXL) has on improving PDT efficacy, it was found that AFXL-assisted PDT was significantly more effective than PDT alone (complete lesion clearance rate of 88% vs. 59% for grade 2–3 AK). The authors concluded that AFXL-assisted PDT has a strong potential in treating AK, especially thick lesions in field-cancerized skin [
104].
Wiegell et al. 2008 [
105], and 2009 [
106] demonstrated that natural daylight photodynamic therapy (DL-PDT) provides similar AK clearance to conventional PDT (c-PDT), is almost painless and is much simpler to perform. DL-PDT involves the application of MAL without occlusion, then 30 min later going outside into daylight for 2 h, after which MAL is removed.
New formulations of ALA/MAL are being trialed, and a recent multi-center study of patients with mild-to-moderate AK to the scalp/face found that PDT with BF-200 ALA (Biofrontera, Leverkusen, Germany) (an ALA nanoemulsion that improves ALA stability and skin penetration) was superior to placebo for complete clearance of 78.2%, and lesion complete clearance rate of 90.4% at 3 months post last PDT [
107].
A self-adhesive, skin colored, thin 5-ALA patch, applied directly to AK lesions without crust removal was superior to cryotherapy in clearing mild/moderate AK [
108]. Side effects of PDT include erythema, itching, edema, exudation, and pain during exposure to light. Local anesthetic nerve blocks or air-cooling for pain relief may be required during PDT if the discomfort is intolerable, with superior analgesia found in those using nerve blocks [
109,
110]. The duration of healing rarely exceeds 10 days. The significant advantage of PDT is high satisfaction with respect to cosmetic outcomes [
98]. The optimal incubation times for ALA/MAL and optimal light sources for PDT are under ongoing investigation. Despite this, it remains an effective lesion- and field-directed therapy.
Ingenol Mebutate: Anti-neoplastic Agent
Ingenol mebutate is a new, Australian-developed topical therapy, a diterpene ester found in the sap of the Euphorbia peplus (petty spurge) plant. It was approved for the treatment of AK in the USA in 2012 and Australia in early 2013, but has not yet been included in the current guidelines. Ingenol mebutate differs from all other current topical treatments in that it has a dual mechanism of action, which may account for the efficacy after a much shorter treatment period. It is currently available in two preparations of 0.015% and 0.05%.
Firstly, there is rapid cellular necrosis through the disruption of the plasma membrane and mitochondrial swelling. This is detectable within 1 h from onset of treatment [
111] and leads to cell death within 24 h in mice [
112].
Secondly, there is a specific neutrophil-mediated, antibody-dependent cellular cytotoxicity (ADCC), which targets any remaining dysplastic epidermal cells [
111]. Antibodies produced by B cells bind to antigens on dysplastic epidermal cells, and these then bind neutrophils which trigger their cytotoxic mechanisms. ROS are released, amongst other lytic agents, which causes the destruction of the dysplastic epidermal cells.
Ingenol mebutate therefore induces both chemo-ablative and immunostimulatory effects after topical application. The rapid destruction of AK lesions means treatment is necessary for only 2 or 3 days: an unusually short duration of treatment for a topical field therapy. A phase IIa study by Siller et al. [
113], with 58 patients was conducted in Australia. Five preselected lesions were treated with ingenol mebutate gel 0.0025, 0.01 or 0.05%, or vehicle gel, on days 1 and 2 (Arm A), or days 1 and 8 (Arm B). The study showed that there were no significant differences in tolerability or efficacy in patients either treated 1 day or 7 days apart. Treatment was well tolerated.
The most common local skin reactions were erythema, scaling, and crusting. The highest dose used (0.05%) achieved highest efficacy, with clinical clearance of 71% of treated lesions. Of all patients treated with 0.05% gel, 67% had clinical clearance of at least four out of five lesions [
113].
In another study, a randomized, double-blind, double-dummy, vehicle-controlled trial was conducted across 22 centers in the USA, where ingenol mebutate was assessed at three dosing regimens for non-facial AK. The three different treatment regimens in that trial were significantly more effective than vehicle in clearing AK. The partial clearance rates ranged from 56% to 75.4% (vs. 21.7% for vehicle), complete clearance rates ranged from 40% to 54.4% (vs. 11.7% for vehicle), and median percentage decrease in baseline AK ranged from 75% to 100% (vs. 0% for vehicle). All arms of active treatment were well tolerated [
114].
Lebwohl et al. [
115] found that complete clearance of AK on the face and scalp treated with ingenol mebutate 0.015% gel for 3 days was 42.2% at the 8-week follow-up visit, and 34.1% for AKs on the trunk and extremities using ingenol mebutate 0.05% gel for 2 days. Local reactions of erythema, scaling, vesiculation, depigmentation, swelling, pruritus and crusting peaks at 4–8 days, and may rarely last up to 30–55 days [
115]. The observational follow-up trial 12 months later showed that a mean of 86% of the number of lesions in the treatment area at baseline were still clear. Approximately half the patients developed a recurrence of one or more lesions in the treated field.
A long-term follow-up study of ingenol mebutate gel for the treatment of actinic keratoses by Lebwohl et al. [
116] was completed in 2013. The results showed that sustained clearance after 12 months was 46.1% for patients treated on the face or scalp with 0.015% gel for 3 consecutive days, and 44.0% for those treated on the trunk/extremities with 0.05% gel for 2 consecutive days. This is considerably better than the 12-month cryotherapy complete clearance rate of 4% as described by Krawtchenko et al. [
63]. The estimated median times to new or recurrent lesions in the treatment area are: 365 days for the face or scalp and 274 days for the trunk or extremities [
116]. The recommended approved therapy is therefore 0.015% gel to the face or scalp for 3 consecutive days, and/or 0.05% gel to the trunk or extremities for 2 consecutive days. Follow-up can usually be recommended at 12 and 9 months, respectively, due to recurrence rates.