Biological Effects of LEO 43204 in Actinic Keratosis

Inclusion Criteria: * 1\. Signed and dated informed consent has been obtained. * 2\. Subjects with at least 5 non-keratotic, clinically typical, visible and discrete AK lesions within a contiguous 250 cm2 area (AK Treatment Area) on the upper extremity, * 3\. Subjects with one additional AK lesion located on the contralateral arm. * 4\. Subjects with an area of normal skin in close proximity to the AK Treatment Area or on the contralateral arm. * 5\. Male and female subjects, 18 years or older. * 6\. Agreement from the patient to allow photographs of the selected treatment area to be taken and used as part of the study data package * 7\. Ability to follow study instructions and likely to complete all study requirements * 8\. Female Subjects must be of either * non-childbearing potential, i.e. post-menopausal or have a confirmed clinical history of sterility (e.g. the subject is without a uterus or has tubal litigation) or, * child-bearing potential\* provided there is a confirmed negative pregnancy test prior to trial treatment to rule out pregnancy. * Female subjects are considered of child-bearing potential unless they have had a hysterectomy or have undergone tubal litigation or have been post-menopausal for at least one year prior to first visit. * 9\. Female subjects of child-bearing potential must be willing to use effective contraception at trial entry and until completion. Effective contraception is defined as follows: * Abstinence (when this is in line with the preferred and usual life style of the subject). * Vasectomised partner (given that the subject is monogamous). * An intrauterine device. * Double barrier method defined as two distinct methods (two actual barrier methods). * Hormonal contraceptive (oral hormonal birth control, oestrogenic vaginal ring, percutaneous contraceptive patches, implants and injectables) for at least one menstrual cycle prior to enrolment. Exclusion Criteria: * 1\. Prior treatment with ingenol mebutate gel in the treatment area within the last 12 months. * 2\. Previously assigned treatment in this clinical trial or previously participated in a clinical trial in the LEO 43204 programme (presently LP0084-68, LP0084-1013, LP0084-1014, LP0084-1015, LP0084-1148, LP0084-1077, LP0084-1193, LP0084-1194, LP0084-1195, LP0084-1196). * 3\. Location of the selected treatment area within 5 cm of an incompletely healed wound or infected area of the skin (excl. study related biopsies) or within 5 cm of a suspected basal cell carcinoma (BCC) or SCC. * 4\. History or evidence of skin conditions other than the study indication that would interfere with the histologically- or biomarker evaluation of the study medication (e.g. eczema, unstable psoriasis, xeroderma pigmentosum). * 5\. Treatment area lesions that have an atypical clinical appearance (e.g. hypertrophic, hyperkeratotic, recalcitrant disease \[had cryosurgery on two previous occasions\] and/or cutaneous horns). * 6\. Known sensitivity or allergy to any of the ingredients of LEO 43204 Gel (e.g. citric acid) * 7\. Clinical diagnosis/history or evidence of any medical condition that would expose a subject to an undue risk of a significant AE or interfere with assessments of safety during the course of the study, as determined by Investigator clinical judgment. * 8\. Anticipated need for in-patient hospitalisation or in-patient surgery during the study period. Note that cosmetic/therapeutic procedures are not excluded if they fall outside of the criteria detailed in the Prohibited Therapies or Medications (listed below). * 9\. Anticipated excessive or prolonged exposure to ultraviolet light (e.g. sunlight) or use of tanning beds for the duration of the study * 10\. Current participation in any other interventional clinical trial * 11\. Subjects who have received treatment with any non-marketed drug product (i.e. an agent which has not yet been made available for clinical use following registration) within the last two months. * 12\. Subjects known to be infected with Human Immunodeficiency Virus (HIV). * 13\. Subject known or, in the opinion of the investigator, is unlikely to comply with the Clinical Study Protocol (e.g. alcoholism, drug dependency or psychotic state). * 14\. Females who are pregnant, of child-bearing potential and wishing to become pregnant during the trial, or are breast feeding. * 15\. Females of child-bearing potential with positive pregnancy test at \[screening or visit 1\]. * 16\. Previous enrolment in this clinical trial. Prohibited Therapies and/or Medications: within 2 weeks prior to the Screening visit * 17\. Cosmetic or therapeutic procedures (e.g. use of liquid nitrogen, surgical excision, curettage, dermabrasion, medium or greater depth chemical peel, laser resurfacing): within 2 cm of the selected treatment areas and within 2 cm of the selected AK lesion outside the treatment area * 18\. Use of acid-containing therapeutic products (e.g. salicylic acid or fruit acids, such as alpha and beta hydroxy acids and glycolic acids), topical retinoids or light chemical peels: within 2 cm of the selected treatment areas and within 2 cm of the selected AK lesion outside the treatment area * 19\. Use of topical salves (non-medicated/non-irritant lotion/cream are acceptable) or topical steroids: within 2 cm of the selected treatment areas and within 2 cm of the selected AK lesion outside the treatment area; artificial tanners: within 5 cm of the selected treatment areas and within 5 cm of the selected AK lesion outside the treatment area Prohibited Therapies and/or Medications: within 4 weeks prior to the Screening visit * 20\. Treatment with immunomodulators (e.g. azathioprine), cytotoxic drugs (e.g. cyclophosphamide, vinblastine, chlorambucil, methotrexate, podophyllin, camptothecin) or interferon/interferon inducers. * 21\. Treatment with systemic medications that suppress the immune system (e.g. cyclosporine, prednisone, methotrexate, infliximab). * 22\. Treatment/therapy with UVB Prohibited Therapies and/or Medications: within 8 weeks prior to the Screening visit * 23\. Treatment with 5-FU, imiquimod, diclofenac, or photodynamic therapy: within 2 cm of the selected treatment areas and within 2 cm of the selected AK lesion outside the treatment area Prohibited Therapies and/or Medications: within 6 months prior to the Screening visit * 24\. Use of systemic retinoids (e.g. isotretinoin, acitretin, bexarotene)