First Step into the Gene Therapy Market

LEO Pharma, a Danish dermatology-focused pharmaceutical company, has taken its first step into the gene therapy field by acquiring Replay, a U.S.-based gene editing biotech company. LEO Pharma will pay an upfront payment of $50 million (USD 50M) to acquire Replay's entire gene therapy technology and preclinical-stage pipeline of treatments for rare skin diseases. This acquisition is part of LEO Pharma's long-term strategy to diversify its business structure, which has traditionally focused on conventional pharmaceuticals, into next-generation modalities such as gene therapy. In particular, the integration of the research team and assets of Telaria, Replay's dermatology-focused subsidiary, is expected to significantly strengthen LEO Pharma's capabilities in rare diseases.

Technological Value of Next-Generation HSV-1 Platform

Replay's core technology is its proprietary 'synHSV' platform, which can safely and effectively deliver large genes. The widely used adeno-associated virus (AAV) vector currently has a gene loading capacity of only 4.7 kb, which is very small and cannot accommodate large genes. In contrast, Replay's herpes simplex virus (HSV)-based vector has a loading capacity up to 30 times larger than AAV, allowing it to fully transport large skin structural protein genes. This next-generation vector technology has the potential to expand beyond skin diseases to treat a variety of genetic diseases in the future.

Therapeutic Opportunity for Target Indication RDEB

The key pipeline that LEO Pharma is targeting in this transaction is a preclinical-stage topical gel treatment for recessive dystrophic epidermolysis bullosa (RDEB). RDEB is a rare and intractable disease in which mutations in the collagen type VII alpha 1 (COL7A1) gene cause severe skin lesions throughout the body, even with minor friction. Replay's treatment works by directly applying an HSV-1 vector loaded with the missing COL7A1 gene to the skin, inducing the synthesis of normal collagen protein. This provides a fundamental therapeutic opportunity to regenerate skin structure, going beyond the symptom relief provided by existing treatments.

Competition with Leading Companies and Market Analysis

Currently, the global RDEB market is dominated by Krystal Biotech's Vyjuvek, which was first approved in 2023. Vyjuvek is also a topical gene therapy based on an HSV-1 vector, and it generated net sales of $389.1 million (USD 389.1M) in 2025, demonstrating its strong commercial potential. Through this acquisition, LEO Pharma will not only enter the already proven high-growth rare skin disease market but also acquire a platform with more advanced loading capacity and ease of re-administration. It plans to use this as a weapon to shake up the market share of the leading company and secure a foothold in the market.

Financial Structure and Development Risk Analysis

LEO Pharma has paid an upfront payment of $50 million for this transaction and has agreed to provide additional milestone payments based on future development stages and single-digit royalties upon commercial success. Although this pipeline is in the preclinical stage, there is a financial risk that the probability of final approval is low and that a long development period will be required. However, considering the high pricing policy and exclusive sales rights granted upon approval, which are typical of rare disease drugs, the expected return on investment is very high. Investors should pay close attention to the timing of entry into Phase 1 clinical trials and the demonstration of clinical superiority compared to the leading drug.