Ingenol Mebutate: An Introduction

Robyn S. Fallen, BHSc and Melinda Gooderham MD, MSc, FRCPC

Skin Centre for Dermatology and Skin Laser Clinic, Peterborough, ON, Canada

ABSTRACT

The incidence of nonmelanoma skin cancer continues to increase. While surgical excision remains the mainstay of treatment, growing demand from patients for effective, tissue-sparing approaches with good cosmetic results has led to the development of novel therapeutic agents. Several studies have reported on the safety and efficacy of topical ingenol mebutate gel, a derivative of the plant Euphorbia peplus, in the treatment of actinic keratosis and superficial basal cell carcinoma. An understanding of the history, mechanism of action, and recent trial evidence for this emerging therapy can assist physicians in counseling patients on available treatment options and in selecting appropriate therapy.

Key Words:
actinic keratosis, ingenol mebutate, PEP005

Introduction

Nonmelanoma skin cancer is the most common type of cancer in humans.¹ While burgeoning incidence in North America has been partially attributed to improved skin cancer surveillance and detection, increased outdoor activity and sun exposure along with changing clothing styles are also implicated in this growing burden of disease.² Although surgical excision remains the mainstay of treatment, there exists demand from patients for medical alternatives that will remove the lesion while achieving a good cosmetic outcome.³ Despite increasing recognition of the role of prevention, including sun avoidance and sun protection through appropriate clothing and use of sunscreen, there remains a need for treatment of existing lesions. It has been recognized that actinic keratoses may occur as a single lesion or affect an entire region of skin (field cancerization).⁴ These lesions can progress to squamous cell carcinoma (SCC) but risk for individual lesions cannot be stratified clinically, supporting a role for active management of actinic keratoses in addition to that of basal cell, intraepidermal, and squamous cell carcinomas.⁴

This search for non-surgical and tissue-sparing treatments has led to the development of novel, non-invasive modalities. Further, with increasing mainstream media attention advocating the use of botanical extracts in both the cosmetic and pharmaceutical industries, the availability of controlled clinical trials is an important consideration in providing education and treatment selection for patients.⁵ Awareness of the history, mechanisms, and evidence for ingenol mebutate, formerly known as PEP005, as an emerging therapy for nonmelanoma skin cancer can assist physicians in counseling patients on available treatment options and in selecting appropriate therapy.

History

Ingenol mebutate (ingenol-3-angelate) is an extract from the plant Euphorbia peplus (E. peplus). More commonly known as the Spurge family, E. peplus belongs to the Euphorbiaceae, one of the most important medicinal families of plants with a long tradition.⁶ Comprised of over 3000 species within 200 genera, Euphorbiaceae is one of the largest families of flowering plants with species distributed over most of the globe.⁶ All euphorbias contain a natural irritant latex, which has been used for medicinal purposes since the time of recorded history.⁶ In more recent history, case reports have documented the use of wild E. peplus in the treatment of basal cell carcinoma,⁷ and participants in an Australian community survey of skin cancer and solar keratosis using naturopathic home remedies unanimously considered E. peplus to be an effective treatment for these lesions.⁸ This long experience of community use with few documented side-effects elicited the initiation of clinical trials assessing the potential of topical E. peplus for nonmelanoma skin cancer.⁹

Mechanism of Action

Several mechanisms have been identified to explain the chemotherapeutic effects of ingenol mebutate, a hydrophobic diterpene ester. Preclinical electron microscopy showed that treatment of a murine model with ingenol mebutate, both in vitro and in vivo, rapidly caused swelling of mitochondria of dysplastic keratinocytes and cell death by primary necrosis.¹⁰ Importantly, by induction of primary necrosis, it is unlikely to have its activity compromised by the development of apoptosis resistance in tumor cells.¹¹

Also of note, ingenol mebutate was observed to promote healing as well as restoration of normal clinical and histologic morphology.¹⁰ The inflammatory response generated by topical application, a predominantly neutrophilic infiltration, has been attributed to protein kinase C activation, and this activity may contribute to effective wound healing.^10,12,13 Protein kinase Cmediated neutrophilic cutaneous infiltrates are the response to the chemokine milieu generated by activated keratinocytes.¹⁴ The neutrophil infiltrate helps prevent relapse of the treated tumor by mediating antibody-dependent cellular cytotoxicity against residual tumor cells.^15,16 The immunostimulatory properties of ingenol mebutate have also been demonstrated to produce regression of smaller, distant untreated tumors.¹⁵

In addition to this induced neutrophilia, the protein kinase C pathway activation of ingenol mebutate also targets and damages the subepidermal intrinsic tumor vasculature.¹⁷ Further preclinical murine modeling has demonstrated that, through P-glycoprotein-mediated absorptive drug transport, ingenol mebutate can pass the stratum corneum barrier and penetrate to exert its pharmacological effects in the dermis and hypodermis.¹⁷

Efficacy

Several controlled studies have shown ingenol mebutate to be effective in the treatment of nonmelanoma skin cancer. Ramsay and colleagues evaluated the efficacy of E. peplus sap, containing approximately 100-300 µg mL^-1 ingenol mebutate.⁹ Patients in this phase I/II clinical study had at least one histologically confirmed basal cell carcinoma (BCC), intraepidermal carcinoma (IEC), or squamous cell carcinomas (SCC). The inclusion criteria accepted patients who were deemed unsuitable for surgical treatments (due to nature or site of lesion, age, or anticoagulant use) or who had failed previous treatments (including surgery, fluorouracil 5% cream, and/or liquid nitrogen) and refused surgical treatment. Thirty-six patients with a total of 48 lesions were treated topically once-daily for 3 consecutive days. The complete clinical response rate, defined as the absence of tumor on clinical exam 1 month following treatment, was 82% (n=28) for BCC, 94% (n=16) for IEC, and 75% (n=4) for SCC.⁹ Most common treatment site reactions included dry skin desquamation, patchy moist desquamation, and/or erythema. In over 60% of patients these skin reactions returned to normal within 1 month, with only mild erythema persisting for an average of 4 months in the remaining patients. Pain was rated from none to mild by 80% of participants receiving treatment with ingenol mebutate. This clinical study affirming the community experience with E. peplus sap provides support for further clinical development of ingenol mebutate for the treatment of basal cell, intraepidermal, and squamous cell carcinomas.

In a phase IIa randomized, double-blind, vehicle-controlled study conducted by Siller and colleagues, the safety of two applications of ingenol mebutate gel was investigated.¹⁸ In 58 patients with biopsy-confirmed actinic keratoses, lesions were treated with ingenol mebutate gel 0.0025%, 0.01% or 0.05%, or vehicle gel. No significant difference in tolerability was found between the gels with active ingredient or with application on days 1 and 2 as compared to days 1 and 8. There were no treatment-related serious adverse events. Local skin reactions consisting of mild to moderate erythema, flaking/scaling/dryness, and scabbing/ crusting were common but returned to baseline within 1 month of treatment cessation. There were no cases of scarring or abnormal lesion proliferation. Although this study also suggested a favorable clinical outcome with the treatment group, due to the limitations of small sample size, this efficacy data should be interpreted with caution.¹⁸

Anderson and colleagues conducted a phase IIb randomized, double-blind, vehicle-controlled study to further explore safety, efficacy, and tolerability.¹⁹ The trial compared ingenol mebutate 0.025% gel applied once-daily for 3 consecutive days or 0.05% gel applied once-daily for 2 or 3 consecutive days as field-directed therapy for an area containing 4-8 clinically typical, discrete actinic keratoses. The severity of local skin reactions in the active treatment groups followed a dose-dependent pattern, peaked between days 3 and 8, and largely resolved by day 15. Compared to vehicle, assessment at day 57 showed a statistically significant difference in partial clearance (56.0% to 75.4% from lowest to highest dosage) and for complete clearance (40.0% to 54.4% from lowest to highest dosage) with ingenol mebutate gel.¹⁹

Phase III trials have recently been undertaken evaluating topical ingenol mebutate gel at 0.05% for the treatment of actinic keratoses located on the trunk and extremities, and 0.015% for the face and scalp, although this data is not yet available.^20-23

Ingenol mebutate is also in clinical development as a short-course treatment for superficial basal cell carcinoma (sBCC). A phase IIa study conducted by Siller and colleagues evaluated the safety of two applications of topical ingenol mebutate gel at concentrations of 0.0025%, 0.01%, or 0.05% on histologically confirmed sBCC.²⁴ Of the 60 patients, adverse effects were low with no serious events. Four patients experienced severe local skin reactions resulting in only one application of medication for two patients and a delay in second application by 1 week for an additional two patients. Although the study population was small, histological clearance rates favored the treatment groups and served as a secondary endpoint.²⁴ A current phase II study will further assess safety and efficacy of ingenol mebutate in the treatment of sBCC.²⁵

Safety and Adverse Events

Available clinical data studying ingenol mebutate over the course of weeks to months has shown a favorable safety profile and has not demonstrated any severe adverse events. Mild to severe local skin reactions have been the most common treatment-related events and are not unexpected given the mechanism of action of both primary necrosis and concurrent inflammation.^9,18,19,24

However, case reports have documented side-effects from accidental ocular exposure to the sap of E. peplus.^26,27 Initial symptoms of ocular exposure were generally stinging pain with blurred vision. Clinical findings varied from mild epithelial keratoconjunctivitis to a severe keratitis, but with supportive management, all signs and symptoms resolved by 1-2 weeks.²⁶ Although such effects of ingenol mebutate gel have not been widely reported, caution with ocular exposure is likely warranted.

Conclusion

While not fully established, the efficacy of ingenol mebutate appears to be comparable with other non-surgical modalities.^28-30 Its use is associated with low toxicity, it is relatively well tolerated by patients, and resultant cosmesis is advantageous. Further, the 2-3 day course of therapy is likely to be viewed favorably by patients in terms of a shorter overall treatment time compared to other available topical therapies and this may show benefits with respect to enhancing adherence. Further data regarding efficacy and direct comparison to currently available therapies will be important in the assessment of ingenol mebutate gel and its role in the treatment of actinic keratoses and nonmelanoma skin cancer. Longitudinal studies will also be needed to confirm clinical cure over time, identify recurrence risk, and elucidate longer-term safety data.

References

1. Galiczynski EM, Vidimos AT. Nonsurgical treatment of nonmelanoma skin cancer. Dermatol Clin 2011 Apr;29(2):297-309.
2. Patel RV, Frankel A, Goldenberg G. An update on nonmelanoma skin cancer. J Clin Aesthet Dermatol 2011 Feb;4(2):20-7.
3. Madan V, Lear JT, Szeimies RM. Non-melanoma skin cancer. Lancet 2010 Feb 20;375(9715):673-85.
4. Fuchs A, Marmur E. The kinetics of skin cancer: progression of actinic keratosis to squamous cell carcinoma. Dermatol Surg 2007 Sep;33(9): 1099-101.
5. Reuter J, Merfort I, Schempp CM. Botanicals in dermatology: an evidencebased review. Am J Clin Dermatol 2010;11(4):247-67.
6. Appendino G, Szallasi A. Euphorbium: modern research on its active principle, resiniferatoxin, revives an ancient medicine. Life Sci 1997;60(10):681-96.
7. Weedon D, Chick J. Home treatment of basal cell carcinoma. Med J Aust 1976 Jun 12;1(24):928.
8. Green AC, Beardmore GL. Home treatment of skin cancer and solar keratoses. Australas J Dermatol 1988;29(3):127-30.
9. Ramsay JR, Suhrbier A, Aylward JH, et al. The sap from Euphorbia peplus is effective against human nonmelanoma skin cancers. Br J Dermatol 2011 Mar;164(3):633-6.
10. Ogbourne SM, Suhrbier A, Jones B, et al. Antitumor activity of 3-ingenyl angelate: plasma membrane and mitochondrial disruption and necrotic cell death. Cancer Res 2004 Apr 15;64(8):2833-9.
11. Ivanov VN, Bhoumik A, Ronai Z. Death receptors and melanoma resistance to apoptosis. Oncogene 2003 May 19;22(20):3152-61.
12. Serova M, Ghoul A, Benhadji KA, et al. Effects of protein kinase C modulation by PEP005, a novel ingenol angelate, on mitogen-activated protein kinase and phosphatidylinositol 3-kinase signaling in cancer cells. Mol Cancer Ther 2008 Apr;7(4):915-22.
13. Lee WY, Hampson P, Coulthard L, et al. Novel antileukemic compound ingenol 3-angelate inhibits T cell apoptosis by activating protein kinase Ctheta. J Biol Chem 2010 Jul 30;285(31):23889-98.
14. Wheeler DL, Ness KJ, Oberley TD, et al. Protein kinase Cepsilon is linked to 12-O-tetradecanoylphorbol-13-acetate-induced tumor necrosis factor-alpha ectodomain shedding and the development of metastatic squamous cell carcinoma in protein kinase Cepsilon transgenic mice. Cancer Res 2003 Oct 1;63(19):6547-55.
15. Le TT, Gardner J, Hoang-Le D, et al. Immunostimulatory cancer chemotherapy using local ingenol-3-angelate and synergy with immunotherapies. Vaccine 2009 May 18;27(23):3053-62.
16. Challacombe JM, Suhrbier A, Parsons PG, et al. Neutrophils are a key component of the antitumor efficacy of topical chemotherapy with ingenol-3- angelate. J Immunol 2006 Dec 1;177(11):8123-32.
17. Li L, Shukla S, Lee A, et al. The skin cancer chemotherapeutic agent ingenol- 3-angelate (PEP005) is a substrate for the epidermal multidrug transporter (ABCB1) and targets tumor vasculature. Cancer Res 2010 Jun 1;70(11): 4509-19.
18. Siller G, Gebauer K, Welburn P, et al. PEP005 (ingenol mebutate) gel, a novel agent for the treatment of actinic keratosis: results of a randomized, doubleblind, vehicle-controlled, multicentre, phase IIa study. Australas J Dermatol 2009 Feb;50(1):16-22.
19. Anderson L, Schmieder GJ, Werschler WP, et al. Randomized, double-blind, double-dummy, vehicle-controlled study of ingenol mebutate gel 0.025% and 0.05% for actinic keratosis. J Am Acad Dermatol 2009 Jun;60(6):934-43.
20. A multi-center, randomized, parallel group, double-blind, vehicle-controlled study to evaluate the efficacy and safety of PEP005 (ingenol mebutate) gel, 0.05%, in patients with actinic keratoses on non-head locations (region-Ib). In: ClinicalTrials.gov NLM Identifier: NCT00942604. Available from: http://clinicaltrials.gov/ct2/show/NCT00942604. Accessed: August 31, 2011.
21. A multi-center, randomized, parallel group, double-blind, vehiclecontrolled study to evaluate the efficacy and safety of PEP005 (ingenol mebutate) gel, 0.015% in patients with actinic keratoses on the head (face or scalp) (region-IIa). In: ClinicalTrials.gov NLM Identifier: NCT00916006. Available from: http://clinicaltrials.gov/ct2/show/NCT00916006. Accessed: August 31, 2011.
22. A multi-center, randomized, parallel group, double-blind, vehiclecontrolled study to evaluate the efficacy and safety of PEP005 (ingenol mebutate) gel, 0.015% in patients with actinic keratoses on the head (face or scalp) (region-IIb). In: ClinicalTrials.gov NLM Identifier: NCT00915551. Available from: http://clinicaltrials.gov/ct2/show/NCT00915551. Accessed: August 31, 2011.
23. A multi-center, open-label study to evaluate the safety and efficacy of PEP005 (ingenol mebutate) gel, 0.05% in patients with actinic keratoses on non-head locations (trunk and extremities). In: ClinicalTrials.gov NLM Identifier: NCT00917306. Available from: http://clinicaltrials.gov/ct2/show/ NCT00917306. Accessed: August 31, 2011.
24. Siller G, Rosen R, Freeman M, et al. PEP005 (ingenol mebutate) gel for the topical treatment of superficial basal cell carcinoma: Results of a randomized phase IIa trial. Australas J Dermatol 2010 May;51(2):99-105.
25. PEP005 Gel – Evaluation of the safety and efficacy of ingenol mebutate gel on a superficial basal cell carcinoma on the trunk or extremities. In: ClinicalTrials.gov NLM Identifier: NCT01325688. Available from: http://clinicaltrials.gov/ct2/show/NCT01325688. Accessed: August 31, 2011.
26. Eke T, Al-Husainy S, Raynor MK. The spectrum of ocular inflammation caused by euphorbia plant sap. Arch Ophthalmol 2000 Jan;118(1):13-6.
27. Lam TSK, Wong OF, Leung CH, et al. A case report of ocular injury by Euphorbia plant sap. Hong Kong J Emerg Med 2009 Oct;16(4):267-270.
28. Rowe DE, Carroll RJ, Day CL Jr. Prognostic factors for local recurrence, metastasis, and survival rates in squamous cell carcinoma of the skin, ear, and lip. Implications for treatment modality selection. J Am Acad Dermatol 1992 Jun;26(6):976-90.
29. Thissen MR, Neumann MH, Schouten LJ. A systematic review of treatment modalities for primary basal cell carcinomas. Arch Dermatol 1999 Oct; 135(10):1177-83.
30. Love WE, Bernhard JD, Bordeaux JS. Topical imiquimod or fluorouracil therapy for basal and squamous cell carcinoma: a systematic review. Arch Dermatol 2009 Dec;145(12):1431-8.