An RFD assigns jurisdiction, not requirements

A combination product built from a drug and a device constituent doesn't get to pick which FDA center reviews it by picking whichever label sounds closest. This blog has already traced what happens once that center is settled — which quality-system provisions a drug-primary system still has to borrow from the device side — but that post starts from an answer already in hand. The question of which center gets to give that answer runs through a separate mechanism, 21 CFR Part 3's Request for Designation process, and it resolves on a narrower basis than most sponsors expect going in.

The test is one mode of action, not a general impression

21 CFR 3.2 defines primary mode of action as the single mode of action of a combination product that provides its most important therapeutic action — the mode expected to make the greatest contribution to the product's overall intended therapeutic effects. That's a narrower question than “which constituent part is the real innovation here” or “which one costs more to manufacture.” A device-led delivery system wrapped around a familiar drug can still land with the drug side if the therapeutic effect traces back to the drug's own action; a novel device that happens to elute an approved compound can land with the device side on the same logic run the other way. The determination follows the therapeutic mechanism, not the parties' own sense of which half of the product they built first.

When the mode can't be determined, an algorithm decides — not a preference

Some combination products genuinely resist the test: two modes of action that contribute independently, with neither clearly subordinate to the other. For that case, 21 CFR 3.4 supplies a fallback algorithm rather than leaving the sponsor to argue for whichever center it would rather deal with. The assignment first looks to how FDA has classified other combination products that raise similar types of safety and effectiveness questions; absent a clear precedent, it goes to whichever agency component has the most relevant expertise to address the product's most significant safety and effectiveness questions. A sponsor invoking the algorithm still has to identify the comparable products and make the expertise argument — the algorithm structures the reasoning, it doesn't excuse having to make one.

A Pre-RFD gets a read of the room; only the RFD binds anyone

FDA's Office of Combination Products offers two distinct ways to ask this question, and they carry different weight. A Pre-RFD, most recently reissued in a 2025 draft guidance, gets informal, non-binding feedback on regulatory identity and likely center assignment — useful for a sponsor still shaping the product, but not a determination either side is committed to later. A formal Request for Designation under 21 CFR 3.7 is the instrument that actually binds: once FDA issues its letter of designation, both the sponsor and the agency are committed to the assignment it states. Treating a Pre-RFD's informal read as if it settled the question the way a formal RFD does is the same mistake this blog has already flagged in 513(g)'s narrower cousin: the informal channel is genuinely useful, and genuinely not the same commitment as the formal one.

The 60-day clock defaults to the sponsor if FDA misses it

A formal RFD has to state the sponsor's own recommended agency component and the basis for it, alongside a description of every mode of action the product presents. From the filing date, FDA's product jurisdiction officer has 60 days to issue a letter of designation naming the center with primary jurisdiction and any consulting components. That deadline isn't advisory: if the officer doesn't issue a designation within the 60 days, the sponsor's own recommended center becomes the designation by default. Few FDA review clocks hand the sponsor's own position that much weight when the agency runs late — it's a real forcing function, and a sponsor that files a weak or unsupported recommendation is gambling that a default it wouldn't have chosen carefully is still the one it might end up with.

Where this meets the rest of the file

The RFD answers one question and stops: which center holds primary jurisdiction, and which others consult. It says nothing about what that center will then require — that's a separate determination, and for a drug-primary combination product it runs straight into the borrowed quality-system provisions Part 4 still layers on top once jurisdiction is no longer in question. A jurisdiction memo that documents the PMOA analysis, the algorithm reasoning where it applies, and the RFD's own binding designation — kept separate from the downstream GMP-mapping question it unlocks — is previewed in the launch catalog. If your program runs this determination differently, the shelf takes that correction directly.