3.2.S.2.3 Control of Materials: The Starting Material Justification That FDA and EMA Scrutinize Most

“The starting material designation for a new drug substance is among the most consequential regulatory decisions in CMC development. It defines the boundary of GMP applicability, the scope of impurity control, and the breadth of the specification set. FDA and EMA apply different but equally demanding evidentiary standards — and a starting material justification that satisfies one may require significant supplementation for the other.”

I have spent two decades writing and reviewing 3.2.S.2.3 sections for NDA, ANDA, and BLA submissions across multiple therapeutic classes and synthetic routes. In that time, the starting material justification has evolved from a brief narrative paragraph to one of the most technically demanding components of the drug substance CTD section. The reason is straightforward: both FDA and EMA have become increasingly sophisticated in their understanding of how starting material selection is used — and misused — to minimize GMP scope, limit the impurity specification burden, and reduce the analytical characterization package. ICH Q11’s definition of a starting material is frequently misapplied to serve commercial rather than scientific rationales, and FDA and EMA reviewers are specifically trained to challenge exactly that kind of designation.

Understanding what 3.2.S.2.3 must actually demonstrate, and why the evidentiary standard for starting material justification is higher than most applicants anticipate, is the difference between a first-cycle approval and a complete response letter with a six-month clock reset.

WHAT 3.2.S.2.3 MUST DEMONSTRATE ABOUT STARTING MATERIALS, REAGENTS, AND SOLVENTS

The regulatory architecture for 3.2.S.2.3 encompasses three distinct categories of materials: starting materials, reagents and solvents, and auxiliary materials. In terms of reviewer attention and deficiency generation, starting material justification commands the overwhelming share of both. ICH Q11 Section 5.1.1 defines a starting material as “a material used in the synthesis of a new drug substance that is incorporated as a significant structural fragment into the structure of the drug substance.” The three-criteria framework that flows from this definition — significant structural fragment, commercial availability, and adequacy of characterization — appears deceptively simple. Each criterion conceals layers of evidentiary requirements that the 3.2.S.2.3 section must address explicitly.

The significant structural fragment criterion is the most technically demanding and most frequently underaddressed. ICH Q11 Section 5.1.1 does not define what percentage of atoms from the starting material must be retained in the final drug substance, and this ambiguity creates the opening through which inadequate justifications pass internal review before failing agency scrutiny. What the guidance does make clear — and what the ICH Q11 Q&A document (2017) elaborates — is that the structural relationship between the proposed starting material and the drug substance must be documented at the molecular level: which bonds are retained, which atoms are incorporated, and what synthetic transformations occur between the proposed starting material and the final drug substance structure. A justification that states the starting material “contributes the core scaffold of the drug substance” without a molecular overlay diagram showing atom-by-atom retention is not sufficient for FDA or EMA. The reviewer cannot assess the criterion from a qualitative assertion. The documentation must show the work.

The commercial availability criterion is evaluated differently than most sponsors expect. Under the ICH Q11 Q&A document (2017), commercial availability is not simply demonstrated by identifying a single commercial supplier. It requires evidence of broad market availability — meaning multiple suppliers, independent synthetic routes, and market characterization data showing the material is a recognized article of commerce rather than a custom intermediate synthesized by a single contract manufacturer with a single route. When an applicant proposes a starting material for which only one or two suppliers exist and those suppliers are operating under a proprietary synthesis licensed from the innovator, the commercial availability criterion fails on its face. EMA’s Questions and Answers on ICH Q11 (2012) explicitly addresses this scenario, noting that a material produced by a sole-source manufacturer through a dedicated synthesis is more appropriately characterized as an intermediate than a starting material, regardless of its commercial designation in trade documentation.

The adequacy of characterization criterion is where the specification set for the proposed starting material becomes the evidentiary record. Under 21 CFR 314.50(d)(1), the submission must include the specifications and analytical procedures used to test the drug substance and its components. For a proposed starting material, the specification must, at minimum, include identity tests sufficient to confirm the structural assignment, a purity test or assay with a defined acceptance criterion, appearance, water content or loss on drying, residual solvents consistent with ICH Q3C Class designations, and, critically, any impurity limits for known process-related impurities that could carry forward into the drug substance and contribute to the impurity profile assessed against ICH Q3A thresholds. When an impurity present in the starting material is a known or potential mutagenic impurity under ICH M7(R1), the starting material specification must include a specific limit for that impurity calculated from the acceptable intake (AI) for the drug substance, back-propagated through the synthesis to the step at which it is introduced — and the specification must demonstrate that control at the starting material level is sufficient given the purge capacity of the subsequent synthetic steps. A starting material specification that lists only identity and assay without addressing these impurity considerations will generate a deficiency in the specific language: “Starting material specification missing limits for [impurity] — known genotoxic precursor in Step 2.”

The documentation package that accompanies the starting material justification must include, beyond the specification and analytical methods, a representative certificate of analysis from each proposed supplier covering a minimum of two to three commercial lots, structural characterization data including NMR, MS, and IR as appropriate to the complexity of the molecule, and — for any supplier qualified as an alternate source — evidence that the alternate supplier’s material meets the specification through independent analytical testing of representative lots. The FDA’s 2017 Guidance for Industry on ICH Q11 Implementation is unambiguous on supplier qualification: the submission must include a description of the controls applied to the supplier, evidence of GMP status where applicable, and site inspection history relevant to the proposed starting material manufacture. “Provide GMP status, site inspection history, and representative batch CoA data” is a standard deficiency when this package is absent.

THE FDA–EMA DIVERGENCE AND THE EMA REFLECTION PAPER (DRAFT 2014, FINALIZED 2017)

The most significant strategic complication in starting material justification for sponsors pursuing both FDA approval and EMA authorization is that the two agencies apply materially different standards for how early in the synthesis the GMP boundary must be set. ICH Q11 does not establish a universal two-to-four-step acceptance window for starting materials. The justification should address all applicable Q11 general principles, including the position of the proposed starting material in the process, impurity fate and purge, the extent of downstream manufacturing under the applicant’s control, and the adequacy of the overall drug-substance control strategy. EMA’s interpretation has moved in a stricter direction, culminating in the EMA Reflection Paper on Starting Materials for Chemically Synthesised APIs — first published as a draft in 2014 and finalized in 2017.

The EMA Reflection Paper (2017) does not merely restate the ICH Q11 framework — it introduces additional evaluative criteria that effectively require GMP control to commence at an earlier point in the synthesis than FDA would typically require for the same molecule. Specifically, the EMA Reflection Paper establishes that the appropriateness of a proposed starting material must be evaluated not only against the three ICH Q11 criteria, but also against the complexity of the transformation steps between the starting material and the drug substance, the number and nature of chiral centers introduced after the proposed starting material, and the potential for introduction of impurities with structural alerts in the post-starting material synthesis steps. In practice, this means that a starting material justified under the three ICH Q11 criteria at a point four steps before the drug substance may be considered acceptable by FDA and rejected by EMA, which may require the GMP boundary to be set at a step five, six, or seven steps from the final structure depending on the complexity of the intermediate transformations.

This divergence creates a concrete regulatory strategy problem for applicants planning simultaneous or sequential NDA and MAA submissions. A starting material designation optimized to minimize GMP scope for the FDA NDA may be challenged in the EMA scientific assessment as inconsistent with the Reflection Paper (2017) criteria, generating a Day 120 List of Outstanding Issues with the specific language: “EMA considers proposed starting material introduced too late — more appropriate starting point is [earlier intermediate].” Addressing this deficiency requires not only a more robust justification but potentially a re-scoping of the GMP boundary, re-qualification of an earlier intermediate as the starting material, and regeneration of the specification package and supplier documentation for the revised starting material. The timeline and resource implications are substantial, and they are entirely avoidable with a prospective dual-agency assessment at the time of starting material selection during development.

GMP APPLICABILITY AND THE IMPURITY SCOPE CONSEQUENCE

The regulatory significance of starting material designation extends beyond the justification document itself to the impurity control framework that the designation creates. Under ICH Q11, impurities introduced at or after the designated starting material must be characterized, qualified, and controlled in accordance with ICH Q3A. Impurities introduced before the designated starting material are addressed within the starting material specification — subject only to the ICH Q11 control framework, which is less prescriptive than Q3A in terms of identification and qualification thresholds. The practical consequence is that a starting material designated early in the synthesis captures more of the impurity profile within the GMP-controlled ICH Q3A framework, while a starting material designated late in the synthesis relies on the starting material specification to control upstream impurities — with no requirement to characterize or qualify those impurities against Q3A thresholds.

FDA and EMA reviewers understand this dynamic precisely, and it is the reason that a starting material justification that appears to serve commercial rather than scientific rationales receives immediate scrutiny. When the proposed starting material is selected at a point where all structurally complex transformations — stereoselective reactions, ring formation, functional group introduction — have already occurred, and only relatively simple derivatization steps remain under GMP, the reviewer’s assessment is that the GMP boundary has been drawn to minimize impurity qualification obligations rather than to reflect a scientifically appropriate boundary based on the synthetic chemistry. The deficiency language in this scenario is direct: “Proposed starting material is a late intermediate and ICH Q11 justification inadequate.”

XGene Starting Material Justification Package A Four-Document Set

When XGene prepares a 3.2.S.2.3 starting material justification for submission, the package comprises four distinct documents assembled before the justification narrative is drafted:

Document 1 — Structural Fragment Analysis: A molecular overlay diagram showing, at the atom and bond level, which structural elements of the proposed starting material are retained in the final drug substance. The analysis includes the molecular formula contribution of the starting material to the drug substance, a list of bonds retained through the synthesis without cleavage, and a characterization of the structural transformations between the starting material and the drug substance. This document responds directly to the ICH Q11 Section 5.1.1 “significant structural fragment” criterion with visual and quantitative evidence rather than qualitative assertion.

Document 2 — Commercial Availability Assessment: A structured market survey identifying a minimum of three independent suppliers of the proposed starting material with distinct synthetic routes, documentation of trade availability including catalog listings, regulatory precedent for commercial use in pharmaceutical synthesis where available, and a risk assessment addressing the scenario of sole-source disruption. This document addresses the ICH Q11 Q&A (2017) commercial availability criterion and directly anticipates the EMA scrutiny on whether the material is an article of commerce or a custom intermediate.

Document 3 — GMP Applicability Analysis: A step-by-step analysis of the synthesis from the proposed starting material to the drug substance, identifying the purification operations at each step, the impurity purge capacity supported by development data, the ICH M7(R1) assessment of all intermediates for structural alerts, and a comparison of the GMP scope against both FDA and EMA interpretive standards including the EMA Reflection Paper (2017) criteria. This document includes a prospective assessment of whether an earlier starting point would be required for an EMA submission.

Document 4 — Regulatory Precedent Review: A review of publicly available FDA and EMA regulatory precedents — EPAR summaries, FDA product-specific guidance documents, approved NDA/MAA reference information where accessible — for drug substances in the same structural class as the proposed drug substance, documenting the starting material designations accepted for comparable synthetic routes. This document provides the comparative regulatory context that strengthens the justification and allows the applicant to position the proposed designation within the range of accepted precedents.

A 3.2.S.2.3 starting material justification built on this four-document foundation answers the questions FDA and EMA reviewers are trained to ask before those questions become deficiencies.

For your current drug substance starting material designation: does your justification document include a specific structural fragment analysis showing which atoms from the starting material are retained in the drug substance, and have you assessed whether the EMA Reflection Paper (2017) criteria would require an earlier starting point for an EMA submission?

If the answer to either question is anything other than an immediate yes, the starting material designation carries deficiency risk that is worth addressing now rather than in response to a complete response letter or Day 120 List of Outstanding Issues. I would be glad to discuss what a starting material justification gap assessment looks like for a synthesis currently in development or already under agency review.