Repurposed drugs and natural products: Why method of treatment patents are worth the investment

In today’s pharmaceutical landscape, the biotech industry is increasingly identifying new therapeutic uses for known compounds, including previously approved drugs, failed candidates, and naturally occurring products. This form of “repurposing” has emerged as a key innovation strategy in drug development, and in some cases offers a faster and more cost-effective route to new therapies. For example, recent reviews suggest that around one-third of such drug repurposing efforts ultimately achieve regulatory approval, compared with an approximate 10% success rate for new drug applications.^{1, 2}

However, the commercialisation of repurposed drugs and natural products is rarely viable without effective patent protection to attract investment. In many cases, such assets can only be protected through method of treatment (MoT) claims. Despite this, investors are often hesitant to back these opportunities due to the perception that MoT patents are of lower value compared to composition-of-matter patents – viewed as inherently weaker, too narrow, or too easy to design around, with some even going as far as stating they are “not worth the paper they are written on.”

This article challenges that perception and demonstrates that MoT patents are, in fact, highly investible and have underpinned some of the biotech industry’s commercially successful assets.

Method of treatment claims

Composition-of-matter claims are widely regarded as the gold standard in patent protection because they cover the active therapeutic compound per se, regardless of how it is formulated or used. In contrast, MoT claims protect the specific medical use of the compound, and grant the patentee a monopoly over the therapeutic application of the drug, but not the drug per se.

Repurposed drugs or natural products are typically only patentable using MoT claims. This is because although the MoT is presumably new, the repurposed drug or natural product itself is not and therefore a claim directed to the compound per se will either not be novel, or may not be directed to patent eligible subject matter in the case of naturally occurring products. Even where purification or formulation of the compound can support composition claims, those claims are often narrow in scope and as such vulnerable to workarounds.

As a result, while biotech companies working with repurposed or natural compounds may find themselves unable to secure robust composition-of-matter claims per se over the asset per se, they can still seek patent protection for the new therapeutic use of the compounds. Furthermore, even if the treatment of the specified indication is also known, protection can also be sought using MoT claims for other aspects of the treatment, such as additional indications, patient populations, dosage regimens, improved safety profile, modes of administration or combination therapy.

The commercial significance of protecting the use of repurposed drugs and natural products is illustrated by some of the world’s most high profile and profitable medicines, a few of which are highlighted below:

Botox®

A prominent example is Botox®, one of the most successful pharmaceutical franchises built on a natural product. Its commercial strength however has not come from exclusivity over the toxin itself, but from a large and layered patent portfolio covering specific medical uses, dosing regimens, injection sites, and formulations, and has approval for a wide range of uses including chronic migraine, spasticity, dystonia, overactive bladder, hyperhidrosis, and cosmetic wrinkle reduction. Many of these indications were discovered years after the toxin was known, and supported new MoT patents extending market exclusivity. The value of this strategy is evident in Botox’s commercial performance, with the franchise generating approximately USD $6.37 billion in 2025 from cosmetic and therapeutic sales.³

Taxol®

Paclitaxel is a diterpenoid compound originally isolated in the late 1960s from the bark of the Pacific yew. Despite being known for over half a century, the innovators behind Taxol® discovered that paclitaxel could be used as an effective anti‑cancer therapy, particularly for: ovarian cancer, breast cancer, non‑small cell lung cancer, and Kaposi’s sarcoma. Taxol became one of the highest‑grossing chemotherapy drugs of its era, generating billions of dollars in revenue during its period of exclusivity, largely provided for by MoT patents. At its peak, Taxol global sales reached USD $1.5 billion.⁴

Viagra®

Similarly, sildenafil (marketed as Viagra®) was originally developed to treat angina and protected by composition-of-matter patents. It was later repurposed to treat erectile dysfunction which was subsequently protected via MoT patents, which ended up being worth substantially more for the franchise in the United States, with this little blue pill reaching peak global sales of USD $1.14 billion.⁵

Vascepa®

Vascepa® is a pharmaceutical product whose active ingredient is icosapent ethyl, a highly purified omega-3 fatty acid derived from fish oil. Like other naturally occurring compounds, Vascepa derives much of its commercial value from a patent portfolio protecting its therapeutic use, including the treatment of cardiovascular disease in conjunction with other therapies (statins) and severe hypertriglyceridaemia. Vascepa generated $182.8 million in worldwide revenue in 2025⁶, despite ongoing litigation in the United States against a generic manufacturer concerning “skinny labelling” of generic icosapent ethyl for the cardiovascular indication, as discussed further below.

So despite the above, why are investors quick to label such claims of lower commercial value?

Enforcement

This perception that MoT patents are of lower commercial value than composition-of-matter claims is often associated with the view that they are difficult to enforce as infringement can be harder to prove, such as when it occurs and who is the actual offender.

For example and unlike composition-of-matter claims, MoT claims have a more limited scope and often require one or more steps to be performed. The question that arises is who is the direct infringer of such a claim? Depending on how it is written, a doctor or indeed the patient could be the direct infringer, as they are the ones performing the one or more steps required to prove direct infringement. Of course, it would be bad business if patent holders went around suing doctors or their patients. So what options for enforcement are left?

Fortunately, many countries, including Australia, Europe and the United States, have indirect infringement provisions in their patent law that allow patentees to sue marketers or suppliers who facilitate the ultimate infringement of a MoT claim, despite not actually practising the claimed method themselves. However, how this provision is applied will vary country by country. For example, in Australia, a question a Court typically asks is “would any supplier have reason to believe the product would be put to the infringing use?” If the evidence establishes that not an insignificant number of people would be prescribed or recommended by doctors for the patented use – including over the counter products - then there is a strong case for indirect infringement. However, based on recent Court decisions in the United States and Europe, the evidentiary burden for patentees seeking to establish indirect infringement of MoT claims in these markets now appears to be higher, with the Courts in these jurisdictions focussing on whether the supplier has engaged in affirmative conduct designed to bring about the infringing use.

Such suppliers will try to circumvent these provisions by “skinny labelling” the product – that is, carving out the patented use from any label by either reciting non-patented indications and/or disclaiming the patented indication. While skinny labelling can be an important consideration in assessing infringement risk, it is not necessarily determinative. Courts in a number of jurisdictions have demonstrated a willingness to look beyond the product label and consider the surrounding circumstances, including how the product is marketed, prescribed and likely to be used in practice. As a result, a supplier may still be found liable for infringement notwithstanding the use of a skinny label.

Australia

In Australia, this principle was illustrated in Neurim Pharmaceuticals (1991) Ltd v Generic Partners Pty Ltd (No 5) [2024] FCA 360, wherein the Court held that the supply of melatonin infringed MoT claims, notwithstanding the use of skinny labelling to carve out the patented use, because the surrounding circumstances still gave it reason to believe the product would be used for the patented MoT. In particular, the Court held that, although the generic product was labelled for “poor quality sleep”, a reasonable supplier would have appreciated that medical practitioners would still prescribe it for the patented use of “non-restorative sleep”, given the overlap in clinical practice and terminology.

Europe

In Sanofi & Regeneron v Amgen (UPC CFI 505/2024), the Court held that Amgen did not infringe on Regeneron’s MoT claims directed to an antibody product for reducing serum lipoprotein(a) levels in specific patient cohorts. Although Amgen’s Repatha® product insert referred to lowering lipoprotein(a) levels under “pharmacodynamic properties” (rather than “therapeutic indications”), the Court was not satisfied that the product was made for the infringing use, nor that physicians would be likely to prescribe it for that purpose. The decision therefore illustrates the importance of proving the factual circumstances necessary to establish indirect infringement.

United States

Recently, in Hikma Pharmaceuticals USA Inc. v. Amarin Pharma, Inc. (No. 24-889), the Supreme Court concluded that the generic suppliers had not actively induced infringement of Amarin's Vascepa® MoT claims relating to the reduction of cardiovascular risk in statin-treated patients. The Court held that skinny labelling, characterisation of the product as a generic equivalent, and the omission of certain information did not amount to active inducement—that is, did not amount to “affirmative steps” taken by the supplier to encourage infringement.

Despite there being a higher bar to prove indirect infringement in the United States and Europe compared to Australia, this should not fuel the notion that MoT patents lack value. Rather, these decisions demonstrate that enforcement often turns on evidence of how products are marketed, supplied and prescribed in practice. Innovators who understand these principles can structure patent portfolios, labelling strategies and monitoring programs to maximise the value of their MoT protection.

Final comments

For companies investigating new therapies based on repurposed drugs and natural products, MoT patents provide valuable market exclusivity. Although they raise distinct enforcement considerations, MoT patents nonetheless provide an important layer of patent protection and can materially enhance the investibility and commercial attractiveness of an asset to prospective investors and commercial partners.