Affordable Prescriptions for Patients Through Improvements to Patent Litigation Act of 2019 This bill limits in certain instances the number of patents that the manufacturer of a biologic drug can assert in a lawsuit against a company seeking to sell a biosimilar version of that drug. (A biologic drug is produced through natural processes or isolated from natural sources. A biosimilar version is substantially similar to the original biologic, which is the reference product, and is often marketed as a less expensive alternative.) The bill's provisions apply to an existing framework that gives the biosimilar manufacturer an abbreviated path to Food and Drug Administration approval to sell the biosimilar. Specifically, if the biosimilar manufacturer completes certain actions under the framework, such as sharing certain information about its product with the reference product manufacturer, the bill limits the number of certain patents that the reference product manufacturer may assert in a lawsuit, such as patents that were filed more than four years after the reference product received market approval. The limit shall not apply to patents claiming certain methods for using the biologic drug. The court in which the infringement lawsuit is filed may increase the limit if justice so requires or if there is good cause for the increase.