August 2026 · 10 min read
Before your drug candidate enters first-in-human trials, regulatory agencies expect you to have characterised its interactions with a defined set of drug transporters. Miss one, and you risk a clinical hold, a label restriction, or a post-market safety signal that could have been identified in a 96-well plate. Get it right, and you have a clean DMPK package that supports your IND without questions.
This guide covers exactly which transporters FDA, EMA, and ICH M12 require — and which are recommended but not mandatory — along with the cutoff values, assay approaches, and decision points that determine whether in vitro data alone is sufficient or whether a clinical DDI study is needed.
The regulatory landscape — three agencies, one aligned framework
The global regulatory framework for drug transporter DDI assessment is now largely harmonised following the 2022 ICH M12 guideline, which consolidates recommendations from the FDA 2020 DDI guidance and the EMA 2012 DDI guideline. For practical purposes, a transporter package built to ICH M12 standards satisfies FDA, EMA, and PMDA requirements simultaneously.
The key distinction to understand is between mandatory transporters (required for all new molecular entities, NMEs) and context-dependent transporters (required only if specific conditions apply — elimination route, therapeutic indication, co-medication profile).
The mandatory transporter panel — required for all NMEs
The following transporters must be assessed for all NMEs during IND-enabling studies. These studies are recommended by both FDA and EMA drug-drug interaction guidelines to evaluate transporter interactions before going into first-in-human trials.
ABC efflux transporters (vesicle assay)
- P-gp (ABCB1 / MDR1) — intestinal absorption, blood-brain barrier, renal and hepatic efflux. Required by FDA, EMA, ICH M12. Substrate and inhibitor assessment.
- BCRP (ABCG2) — intestinal absorption, hepatic efflux, breast/placental barrier. Required by FDA, EMA, ICH M12. Substrate and inhibitor assessment.
SLC uptake transporters (cell-based assay)
- OATP1B1 (SLC01B1) — hepatic uptake; key determinant of statin and bilirubin pharmacokinetics. Required by FDA, EMA, ICH M12.
- OATP1B3 (SLC01B3) — hepatic uptake; often co-assessed with OATP1B1. Required by FDA, EMA, ICH M12.
- OCT2 (SLC22A2) — renal secretion of cationic drugs; metformin is the clinical probe substrate. Required by FDA, EMA, ICH M12.
- OAT1 (SLC22A6) — renal secretion of anionic drugs; furosemide probe. Required by FDA, EMA, ICH M12.
- OAT3 (SLC22A8) — renal secretion of anionic drugs, often co-assessed with OAT1. Required by FDA, EMA, ICH M12.
- MATE1 (SLC47A1) — renal secretion, apical side; completes the renal cation secretion pathway with OCT2. Required by FDA, EMA, ICH M12.
- MATE2-K (SLC47A2) — renal secretion; kidney-specific isoform of MATE2. Required by FDA, EMA, ICH M12.
Context-dependent transporters — required under specific conditions
The following transporters are not universally required but become mandatory when specific conditions apply to your NME:
| Transporter | Gen | When required | Agency |
|---|---|---|---|
| BSEP | ABCB11 | All NMEs with hepatic elimination, or any in vitro signal of hepatotoxicity. Although FDA 2020 guideline did not explicitly mention BSEP, ICH, EMA and PMDA all recommend investigation to determine if the drug can inhibit BSEP due to the positive correlation between BSEP inhibition and liver toxicity. | ICH M12, EMA, PMDA (not FDA 2020 explicitly) |
| MRP2 | ABCC2 | Signs of cholestasis or conjugated hyperbilirubinemia in preclinical or clinical data. The International Transporter Consortium recommends that new drug candidates are investigated for MRP2 inhibition if signs of cholestasis or conjugated hyperbilirubinemia surface in clinical trials. | EMA, ITC recommendation |
| OCT1 | SLC22A1 | Hepatic uptake relevant drugs; if OCT1 is a major hepatic uptake pathway. EMA recommends; FDA does not require explicitly. | EMA |
| MRP3, MRP4 | ABCC3, ABCC4 | Hepatic basolateral efflux pathway relevant; if MRP2 is inhibited and compensatory routes matter. | Scientific rationale; no explicit mandate |
| OATP1B1/1B3 substrate | SLC01B1/1B3 | Hepatic metabolism or biliary excretion accounts for ≥25% of drug elimination. Examination of whether an investigational drug is a substrate for OATP1B1 and 1B3 should be considered if hepatic metabolism or biliary excretion accounts for greater than or equal to 25% of elimination. | FDA (ICH M12) |
The cutoff values — when in vitro data triggers a clinical study
In vitro inhibition data alone is not always sufficient for regulatory purposes. Each transporter has a defined ratio cutoff: if your compound’s concentration relative to its IC50 exceeds the cutoff, a clinical DDI study is required to confirm or exclude a clinically meaningful interaction.
| Transporter | Inhibitor assessment — concentration used | Cutoff ratio — triggers clinical study if exceeded |
|---|---|---|
| P-gp (intestinal) | Total Cmax after highest therapeutic dose | [I] / IC50 ≥ 0.1 |
| P-gp (systemic) | Unbound steady-state Cmax (Cmax,u) | Cmax,u / IC50,u ≥ 0.02 |
| BCRP (intestinal) | Total Cmax after highest therapeutic dose | [I] / IC50 ≥ 0.1 |
| BCRP (systemic) | Unbound steady-state Cmax | Cmax,u / IC50,u ≥ 0.02 |
| OATP1B1 / 1B3 | Unbound maximum hepatic inlet concentration (Iin,max,u) | R = 1 + (fu,p × Iin,max / IC50) ≥ 1.1 |
| OCT2 / OAT1 / OAT3 | Unbound systemic Cmax (Cmax,u) | Cmax,u / IC50,u ≥ 0.02 |
| MATE1 / MATE2-K | Unbound systemic Cmax | Cmax,u / IC50,u ≥ 0.02 |
| BSEP | Unbound Cmax in hepatocyte | No universal FDA cutoff; EMA: IC50 below clinical exposure triggers monitoring |
If your IC50 is comfortably above the clinical exposure (ratio well below cutoff), the in vitro data alone is sufficient for the IND package — no clinical DDI study required for that transporter. If the ratio approaches or exceeds the cutoff, a clinical study is needed before NDA/BLA submission.
FDA vs. EMA vs. ICH M12 — the key differences
While the three frameworks are largely aligned, a few practically important differences remain:
- BSEP: FDA 2020 guidance does not explicitly list BSEP as a required transporter. ICH M12, EMA, and PMDA do. In practice, BSEP testing is expected for any drug with hepatic involvement — omitting it creates regulatory risk even in FDA submissions, because reviewers apply ITC recommendations.
- OCT1: EMA recommends OCT1 (hepatic uptake, relevant for metformin co-medication); FDA does not explicitly require it. Include in your package if your drug has a hepatic uptake component or targets a metabolic indication where metformin co-medication is likely.
- MRP2: EMA recommends routine MRP2 assessment; FDA does not require it unless there is clinical evidence of cholestasis. Pragmatically: if you are testing BSEP (which you should), adding MRP2 to the vesicle assay panel costs little and eliminates a regulatory question.
- ICH M12 (2022): Consolidates FDA and EMA into a single harmonised guidance. Use ICH M12 as your primary reference for all new IND packages — it is the most current and globally applicable standard.
The complete recommended in vitro transporter panel
Based on ICH M12, FDA 2020 and EMA 2012, here is the practical panel to include in every IND-enabling DMPK package:
| Transporter | Klasse | Assay format | Priority |
|---|---|---|---|
| P-gp (MDR1/ABCB1) | ABC-Efflux | Cell-based (substrate) + vesicular (inhibitor/high Papp) | Mandatory |
| BCRP (ABCG2) | ABC-Efflux | Cell-based (substrate) + vesicular (inhibitor) | Mandatory |
| OATP1B1 | SLC uptake | Transfected cell monolayer | Mandatory |
| OATP1B3 | SLC uptake | Transfected cell monolayer | Mandatory |
| 2. Oktober | SLC uptake | Transfected cell monolayer | Mandatory |
| OAT1 | SLC uptake | Transfected cell monolayer | Mandatory |
| OAT3 | SLC uptake | Transfected cell monolayer | Mandatory |
| MATE1 | SLC efflux | Transfected cell monolayer | Mandatory |
| MATE2-K | SLC efflux | Transfected cell monolayer | Mandatory |
| BSEP | ABC-Efflux | Vesicular transport assay | Strongly recommended (ICH M12, EMA, PMDA) |
| MRP2 | ABC-Efflux | Vesicular transport assay | Recommended (EMA); add if BSEP testing |
| OCT1 | SLC uptake | Transfected cell monolayer | Recommended (EMA); add for hepatic drugs |
What happens if you miss a transporter
Missing a transporter from your IND-enabling package does not automatically result in a clinical hold — but it generates an Information Request from FDA or a List of Questions from EMA that must be answered before proceeding. In practice, this means:
- Months of delay while the missing study is run
- Potential for a label restriction if the missing transporter turns out to be clinically relevant (e.g. a statin interaction via OATP1B1 that was not assessed)
- Post-marketing commitments to run the missing study after approval — adding cost and regulatory burden
The most common omission seen in IND packages: BSEP (missed because FDA 2020 does not explicitly list it), and MATE2-K (often bundled with MATE1 but using a non-specific cell system). Both generate routine regulatory questions.
In-house transporter testing with Cell4Pharma vesicle kits
For the ABC transporter portion of your DMPK package (P-gp inhibitor IC50, BCRP inhibitor IC50, BSEP, MRP2), ready-to-use vesicle kits make in-house testing fast and cost-effective compared to full CRO outsourcing.
SeamlessBio distributes the Cell4Pharma ABC Transporter Vesicle Kit series — validated, ready-to-use inside-out membrane vesicles for IND-enabling inhibitor characterisation:
- BSEP-Vesikel-Kit — DILI risk assessment, EMA/PMDA/ICH M12 recommended
- P-gp-Vesikel-Kit — MDR1/ABCB1 inhibitor IC50
- BCRP-Vesikel-Kit — ABCG2 inhibitor IC50
- MRP2-Vesikel-Kit — ABCC2; hepatic efflux, cholestasis risk
- MRP1–MRP5, MRP8 Vesicle Kits — complete MRP panel for comprehensive screening
- Kontrollvesikel-Kit — non-transfected membrane control for background subtraction
Each kit: 100 reactions, validated probe substrates, ATP regenerating system, full assay protocol and QC documentation. Stored at −80°C; stable 12 months.
→ View the full Cell4Pharma kit portfolio
Summary — the regulatory checklist
Before your IND package is complete, confirm the following for your NME:
- ✅ P-gp — substrate and inhibitor characterised (cell-based + vesicular where needed)
- ✅ BCRP — substrate and inhibitor characterised
- ✅ OATP1B1 and OATP1B3 — inhibitor IC50; substrate if hepatic elimination ≥25%
- ✅ OCT2, OAT1, OAT3 — inhibitor IC50 determined
- ✅ MATE1 and MATE2-K — inhibitor IC50 determined
- ✅ BSEP — inhibitor IC50 assessed (vesicular transport assay)
- ✅ MRP2 — add if BSEP inhibition observed or hepatic signals present
- ✅ Cutoff ratios calculated for each transporter using clinical Cmax
- ✅ Clinical DDI study triggered where ratio exceeds cutoff
Further reading on seamlessbio.de:
- Vesicular Transport vs. Cell-Based vs. ATPase Assay — Which Method?
- BSEP Inhibition & DILI — From Vesicle Assay to High-Throughput Screening
- Cell4Pharma ABC Transporter Vesicle Kit Portfolio
- CRO & CDMO Biologicals Supply Guide
Questions about your transporter panel or kit selection for IND-enabling studies? Contact us at info@seamlessbio.de oder request a quote.
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