Exelixis vs MSN: The Federal Circuit Rejects Polymorph Written Description Challenge Against Cabometyx
By Prashant Kotian, PhD Researcher (Chemistry), ICT Mumbai
Exelixis disclosed only two specific crystalline polymorphs of its blockbuster drug cabozantinib (L)-malate in its patent specification. But its patent claims covered the entire genus of 'crystalline cabozantinib (L)-malate.'
When MSN Laboratories designed a different polymorph for its generic ANDA, it challenged the patent, arguing Exelixis lacked written description for the broader genus because they hadn't mapped out enough structural species to show they possessed the whole group.
The Federal Circuit firmly disagreed. In a precedential decision (*Exelixis, Inc. v. MSN Laboratories*), the court held that disclosing the chemical name, the structural formula, and the crystalline nature of the salt was sufficient to meet the written description requirement for the broad crystalline genus.
Crucially, the court found that a skilled artisan would recognize the inventors possessed the claimed genus of crystalline salts, even without disclosing every possible polymorphic form or enumerating distinct X-ray powder diffraction patterns for uncharacterized variants.
This decision sets an important baseline for pharmaceutical patent portfolios: genus claims for crystalline solid forms can survive Section 112 challenges even when only a subset of polymorphs is explicitly characterized in the specification.
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