Written by Scott Siera, Ph.D. and Eric Furman, Ph.D.

Seven years after Mylotarg® (gemtuzumab ozogamicin) was voluntarily withdrawn from the market, the U.S. Food and Drug Administration has re-approved Mylotarg® product.

Mylotarg® product was first approved by the FDA in 2000 for the treatment of adults with CD33-positive acute myeloid leukemia (AML) who had experienced relapse. However, Mylotarg® product was voluntarily withdrawn from the market in 2010 after a postmarketing trial showed an increased risk of death in a subpopulation treated with Mylotarg® product in combination with chemotherapy.

The new FDA approval is for a lower recommended dosage range and a different administration schedule than the previous approval, and includes uses alone or in combination with chemotherapy. Additionally, the new FDA approval is for a different patient population than before, including adults with newly diagnosed CD33-positive AML, as well as certain younger patients who had experienced a relapse or had not responded to initial treatment.

Mylotarg® product is an antibody-drug conjugate that includes a humanized antibody against the CD33 antigen (a.k.a. Siglec-3, a marker of cells of the myeloid lineage) conjugated to a calicheamicin toxin.

From a patent perspective, this story demonstrates that there can be value in follow-on patent protection covering particular dosing and administration regimens, even if the active ingredient, or other dosing or treatment regimens were previously known. For example, a later-developed dosing or treatment regimen, or a newly-identified patient subpopulation may offer advantages that could not have been expected from earlier clinical or post-marketing trials.

Advantageously, the life of a patent estate may be significantly extended by a later-filed patent based on a new treatment method that was not obvious over previous treatment methods. Moreover, for the first approved commercial marketing of a pharmaceutical or biopharmaceutical product, it is possible to obtain patent term extension (for a portion of the time that a product was in regulatory review) for claims directed to the approved product as well as methods of treatment and methods of manufacturing. Thus, in some situations, patent term extension may be pursued for a later-filed patent so as to extend the patent term well beyond an initial “active ingredient” or first-generation “method of treatment” or “formulation” patent.

However, obtaining follow-on patent protection can often be challenging in the face of previous public disclosures and patent filings. Strategies for protecting new uses of known compounds and improved treatment regimens can benefit from a careful analysis of what was previously disclosed, the new benefits that are achieved (from both a scientific and clinical perspective), and the particular nuances of the patent law.