Blog · DMPK · ICH M12 · Transporter Assays · October 2026 · 9 min read

ICH M12 Transporter Compliance: Which Assays You Need and When

ICH M12 — adopted in 2024 — is now the global harmonised guideline for drug interaction studies. For DMPK scientists, it defines exactly which transporter assays are required, which are discretionary, and what the decision triggers are. This post breaks it down into a practical reference.

What Is ICH M12 and Why It Matters for Transporter Assays

ICH M12 (Guideline on Drug Interaction Studies) was adopted by the ICH Assembly in 2024 and supersedes the previous FDA (2020) and EMA (2013) DDI guidelines as the globally harmonised framework. It is now the primary reference for any IND/NDA/IMPD submission to the FDA, EMA, PMDA (Japan) or Health Canada that includes transporter DDI data.

The key change from the previous regional guidelines: ICH M12 adopts a unified decision framework — a set of explicit mechanistic cut-offs and tiered testing criteria — that replaces the partially contradictory and often ambiguous advice in the regional guidelines. For DMPK labs, this means more predictable testing requirements but also less flexibility to argue edge cases away without data.

The Core Transporter Panel: What Is Always Required

ICH M12 defines a set of nine transporters that must be assessed as potential inhibition targets for every new drug candidate submitted for regulatory approval. These are:

Transporter	Gene	Location	Inhibition Required?	Substrate Required?	Available via Cell4Pharma / ciPTEC
P-gp (MDR1)	ABCB1	Intestine, liver, kidney, BBB	Yes	Yes (if P-gp is a major elimination pathway)	✓ Vesicle Kit
BCRP	ABCG2	Intestine, liver, kidney, BBB	Yes	Yes	✓ Vesicle Kit
OATP1B1	SLCO1B1	Hepatic basolateral	Yes	Yes (if hepatic uptake involved)	Cell-based (HEK293)
OATP1B3	SLCO1B3	Hepatic basolateral	Yes	Yes	Cell-based (HEK293)
OAT1	SLC22A6	Renal basolateral	Yes	Yes (if ≥25% active renal secretion)	✓ ciPTEC-OAT1
OAT3	SLC22A8	Renal basolateral	Yes	Yes	✓ ciPTEC-OAT3
OCT2	SLC22A2	Renal basolateral	Yes	Yes	✓ ciPTEC-OAT1 (endogenous)
MATE1	SLC47A1	Renal apical, liver	Yes	Yes	✓ ciPTEC-OAT1 (endogenous)
MATE2-K	SLC47A2	Renal apical	Yes	Yes	✓ ciPTEC-OAT1 (endogenous)

Additional Transporters: When They Are Triggered

Transporter	Trigger Condition	Assay Format
BSEP (ABCB11)	Hepatobiliary elimination pathway, or DILI risk signal present	Vesicle inhibition assay — ✓ Cell4Pharma BSEP Kit
MRP2 (ABCC2)	Involvement of glucuronide metabolite biliary excretion suspected	Vesicle inhibition assay — ✓ Cell4Pharma MRP2 Kit
OCT1	Hepatic uptake cannot be fully explained by OATP1B1/1B3	Cell-based (hepatocytes or OCT1-HEK293)
OATP2B1	Intestinal absorption faster than passive permeability suggests	Cell-based (OATP2B1-HEK293)
MRP3/4	Renal or hepatic efflux of metabolites	Vesicle assay — ✓ Cell4Pharma MRP3/MRP4 Kits
P-gp (CNS)	CNS indication or compound that may cross the BBB	Bidirectional permeability (MDR1-MDCK or Caco-2)

The ICH M12 Decision Logic: When to Stop Testing

Inhibition screening: the basic R-value cut-off

For each transporter in the core panel, the first in vitro step is an inhibition screen at a single concentration. ICH M12 aligns with the established R-value criterion: if the ratio of the maximum unbound clinical plasma concentration (Cmax,u) to the in vitro IC50 exceeds 0.1 (R = 1 + Cmax,u/IC50 > 1.1 for some transporters), a clinical DDI study is triggered. Below this threshold, clinical interaction risk is considered low.

Substrate assessment: when is active transport clinically relevant?

For substrate studies, ICH M12 applies a pathway contribution threshold: active renal secretion must account for ≥25% of total renal clearance before OCT2/MATE and OAT substrate assessment is considered mandatory. For hepatic uptake transporters (OATP1B1/1B3), the criterion is whether hepatic uptake is a significant elimination pathway. If both conditions are clearly not met, substrate testing can be deferred with adequate mechanistic justification.

The BSEP exception

BSEP is not part of the nine mandatory inhibition targets for all compounds. It is triggered specifically when the compound has a hepatobiliary elimination pathway or when non-clinical findings (elevated bile acids, cholestatic histopathology, hepatocyte vacuolation) suggest a DILI risk. When triggered, BSEP inhibition data must be included in the regulatory package. In DILI-sensitive development programmes, many sponsors run BSEP inhibition screening early in the discovery phase regardless of regulatory trigger, using vesicle kits like the Cell4Pharma BSEP Kit for rapid screening.

Key practical point: ICH M12 explicitly allows in vitro transporter data to replace clinical DDI studies when the mechanistic case is clear and the R-value is below the trigger threshold. This makes high-quality in vitro data — generated with validated assay systems — more important than ever. Weak or under-validated in vitro data no longer provides regulatory cover; it can trigger additional clinical studies.

How Cell4Pharma Kits and ciPTEC Map to ICH M12

SeamlessBio DMPK portfolio — ICH M12 coverage

ABC transporter vesicle kits (Cell4Pharma, distributed by SeamlessBio in DACH): P-gp (ABCB1), BCRP (ABCG2), BSEP, MRP1–5, MRP8 — expressed in HEK293-derived inside-out membrane vesicles. Covers the full efflux transporter inhibition panel required under ICH M12.

ciPTEC renal cell lines (Cell4Pharma): ciPTEC-OAT1 expresses OAT1, OAT3, OCT2, MATE1, MATE2-K, P-gp, BCRP and MRP2/4 endogenously — covering the complete renal transporter substrate panel mandated under ICH M12 in a single cell line. AstraZeneca-validated, FDA/EMA DDI guideline-compliant expression confirmed.

ICH M12 Transporter Kits — In-House or Service

Cell4Pharma ABC transporter vesicle kits and ciPTEC cell lines — available for in-house DMPK screening in DACH. Free test samples available. SeamlessBio is the exclusive DACH distributor for Cell4Pharma.

View DMPK Portfolio Request a Free Test Kit

Frequently Asked Questions

What is the difference between ICH M12 and the previous FDA/EMA DDI guidelines?

ICH M12 is the globally harmonised replacement for the regional FDA (2020 DDI Guidance) and EMA (2013 DDI Guideline). The core transporter panel is largely the same — nine mandatory inhibition targets — but ICH M12 introduces unified mechanistic cut-off criteria, clearer guidance on when substrate testing is triggered, and explicit harmonisation between jurisdictions. For sponsors submitting to multiple regulatory agencies simultaneously, ICH M12 reduces the need to tailor DDI packages per region.

Is BSEP testing required for every drug candidate under ICH M12?

No — BSEP is not part of the nine mandatory transporter inhibition targets that apply to every compound. BSEP testing is triggered when the compound has a hepatobiliary elimination route or when non-clinical findings suggest a DILI risk (e.g. elevated bile acids, cholestatic histopathology). Many sponsors include early BSEP screening anyway as a risk management strategy, using vesicle assays for rapid, low-cost initial assessment.

Can ciPTEC cells be used to satisfy ICH M12 renal transporter requirements?

Yes. ciPTEC-OAT1 expresses OAT1, OAT3, OCT2, MATE1 and MATE2-K — the five renal transporters in the ICH M12 core panel — plus additional efflux transporters (P-gp, BCRP, MRP2, MRP4). This makes ciPTEC-OAT1 capable of generating the renal substrate and inhibition data required under ICH M12 in a single, validated cell system. ciPTEC has been validated by AstraZeneca and cited in peer-reviewed publications that meet current regulatory standards.

What R-value cut-off does ICH M12 use for transporter inhibition screening?

For most transporters in the ICH M12 core panel, the trigger criterion is based on the ratio of maximum unbound plasma concentration (Cmax,u) to the in vitro IC50. The specific cut-offs vary by transporter: for intestinal P-gp and BCRP the criterion uses the intestinal concentration [I] instead of Cmax,u. Always consult the current ICH M12 guideline text and your regional authority’s implementation notes for the exact cut-off values, as these may be refined in regional annexes.

ICH M12 Transporter