42 CFR §423.2310

1. (a)Covered Part D drug coverage requirement. Except as specified in paragraph (b) of this section, in order for coverage to be available under Medicare Part D for applicable drugs of a manufacturer, the manufacturer must do all of the following:
   1. (1)Participate in the Discount Program.
   2. (2)Have entered into and have in effect an agreement described in § 423.2315(b).
   3. (3)Have entered into and have in effect, under terms and conditions specified by CMS, a contract with the TPA.
2. (b)Exception to covered drug coverage requirement. Paragraph (a) of this section does not apply to an applicable drug if CMS has made a determination that the availability of the applicable drug is essential to the health of beneficiaries enrolled in Medicare Part D.