What's the difference between R-PLEX and U-PLEX?

U-PLEX configures custom panels from existing, qualified, linker-coupled antibodies in the MSD catalog — it's a configuration exercise, not development. Timeline: days. R-PLEX develops new MSD assays for targets not in any catalog — it's de novo assay development including antibody screening, pairing, SULFO-TAG labeling, and qualification. Timeline: 8–16 weeks. If your target is already in the U-PLEX catalog, use U-PLEX. If it's not, R-PLEX is the path.

Are custom-developed assays validated for regulatory use?

Custom assays are fit-for-purpose qualified as standard, with qualification parameters documented per your intended use context. For programs supporting regulatory submissions, additional validation scope (GLP, method validation per ICH M10) is available on request. All services are for research and development use; discuss your regulatory requirements during project initiation to ensure appropriate qualification and documentation levels.

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MSD Custom Multiplex Assay Development

R-PLEX · Custom Development

De Novo Assay · SULFO-TAG Labeling · ELISA Conversion

MSD Custom Assay Development & R-PLEX Services

Develop de novo Electrochemiluminescence (ECL) assays for targets not in the V-PLEX or U-PLEX catalogs — including antibody screening and pairing, SULFO-TAG conjugation, assay optimization, and fit-for-purpose qualification — plus ELISA-to-ECL platform conversion for existing validated immunoassays.

Pre-configured V-PLEX and U-PLEX panels cover hundreds of analytes — but the human proteome has thousands. When your target is not in the catalog, R-PLEX builds a new MSD assay from the ground up: antibody pair screening and selection, SULFO-TAG conjugation (with documented label incorporation measured at 455 nm to avoid A280 interference), capture antibody spotting optimization on MULTI-ARRAY plates, and fit-for-purpose qualification. This is fundamentally different from U-PLEX configuration — U-PLEX assembles existing qualified antibodies into custom panels; R-PLEX creates new assays for targets with no existing MSD reagents. For laboratories with established ELISA assays, our ELISA-to-ECL conversion service transfers existing validated antibody pairs to the MSD format — gaining ECL's sensitivity (10–100×), wider dynamic range (3–4+ logs vs. 1–2), and multiplex compatibility while preserving your established reagent investment.

Discuss Your Custom Project Development Process

When Off-the-Shelf Panels Aren't Enough

The MSD catalog — V-PLEX, U-PLEX, and S-PLEX — covers the most commonly studied cytokines, chemokines, growth factors, metabolic hormones, and biomarkers. But research doesn't stop at the catalog boundary. Novel biomarkers, species-specific targets, post-translationally modified proteins, neo-epitopes, anti-idiotypic antibodies for PK/ADA, and emerging therapeutic targets frequently require custom assay development. R-PLEX addresses this through a structured, milestone-driven development process: antibody screening → pairing → labeling → optimization → qualification. The resulting assay runs on the same MSD platform as all V-PLEX and U-PLEX panels — enabling your custom target to be multiplexed alongside catalog analytes in future studies and eliminating the platform-switching variability that complicates cross-study comparison.

 When to Choose Custom Development
Target not in V-PLEX or U-PLEX catalog: novel biomarker, emerging therapeutic target, or species-specific analyte not covered by existing panels. R-PLEX development from antibody screening through qualification.
PK assay for a novel biologic: anti-idiotypic or anti-drug antibody pairs developed and qualified for pharmacokinetic quantification of your specific drug candidate, with wide dynamic range (3–4+ logs) for Cmax-to-trough coverage.
ADA assay for immunogenicity testing: bridging format ADA assay using your drug as both biotinylated capture and SULFO-TAG-labeled detection reagent — with acid-dissociation protocol optimized for drug tolerance.
ELISA-to-ECL conversion: existing validated ELISA antibody pairs transferred to the MSD MULTI-ARRAY format — capture antibody spotted onto plates, detection antibody conjugated with SULFO-TAG. MSD ELISA Conversion Packs provide uncoated plates for immobilizing unlabeled capture antibodies.
Post-translational modification or neo-epitope assays: phospho-specific, cleavage-specific, or conformation-specific antibodies requiring custom assay configuration not available in standard catalogs.

R-PLEX Custom Development vs. U-PLEX Configuration

	U-PLEX (Configuration)	R-PLEX (Development)
Target Must Be in Catalog?	Yes — linker-coupled antibodies must already exist	No — any target with suitable antibody pairs
Timeline	Days — antibodies assembled from inventory	8–16 weeks — screening, pairing, optimization
What You Provide	Target list — we check catalog availability	Antibodies, or target protein for screening, or both
Assay Performance	Pre-characterized per individual target	Characterized during development — sensitivity, dynamic range, specificity documented
Cost Structure	Per-plex panel pricing	Milestone-based development fee + per-sample analysis
Multiplex Potential	2–10 analytes per well	Custom target can be multiplexed with U-PLEX catalog analytes in future studies
Best For	Custom combinations of existing catalog analytes	Novel targets, PK/ADA assays, species-specific targets not in catalog, ELISA conversions

R-PLEX is MSD's product name for de novo assay creation; it is distinct from U-PLEX panel configuration. All custom development is for research use.

Custom Assay Development Process

Phase 1: Feasibility & Screening

Target & reagent assessment: submit your target and available reagents (antibodies, recombinant protein). We evaluate feasibility and provide a milestone-based project plan — typically within 1–2 weeks. No obligation.
Antibody screening: if suitable antibody pairs are not yet identified, we screen candidate antibodies for binding to target and select optimal capture-detection pairs based on sensitivity, specificity, and matrix compatibility.
Preliminary range-finding: initial standard curves to establish the approximate working range before committing to full optimization.

Phase 2: Development & Qualification

SULFO-TAG conjugation: detection antibody conjugated with SULFO-TAG NHS-ester. Label incorporation measured at 455 nm (not 280 nm — SULFO-TAG absorbs strongly at 280 nm, interfering with A280 protein measurement). Typical challenge ratio: 12:1 (SULFO-TAG:antibody). Protein quantitation by BCA assay with IgG standards.
Biotin conjugation: capture antibody labeled with biotin for streptavidin-plate immobilization. Goal: 2–4 biotin labels per antibody for optimal surface presentation without steric hindrance.
Assay optimization: antibody concentrations, incubation times, sample dilution, matrix compatibility, and minimum required dilution (MRD) established.
Fit-for-purpose qualification: standard curve range, LOD/LLOQ, intra- and inter-assay precision, dilution linearity, spike recovery in relevant matrix, and selectivity. Documentation provided for your intended use context.

ELISA-to-ECL Platform Conversion

Antibody Transfer & Conjugation

Your validated capture antibody is immobilized on MSD MULTI-ARRAY plates (using MSD ELISA Conversion Packs for uncoated plate formats). Your detection antibody is conjugated with SULFO-TAG NHS-ester with documented label incorporation. Antibody specificity and affinity are preserved — the detection chemistry changes, not the binding reagents.

Performance Improvement

ECL conversion typically improves sensitivity 10–100× over colorimetric or fluorescent ELISA. The low background of electrical excitation (zero autofluorescence) and high binding capacity of the carbon electrode surface extend the lower limit of detection without changing the antibody pair. Dynamic range expands from 1–2 logs to 3–4+ logs.

Multiplex Compatibility

Once converted, your custom assay can be multiplexed with V-PLEX or U-PLEX catalog analytes — enabling your target to be measured alongside standard biomarker panels from the same 25 µL sample well. Eliminates the platform-switching variability that would result from running ELISA and MSD on separate instruments.

Custom Development Service Details

Service Package

Item	Full Service	Notes
Feasibility Assessment	Complimentary	1–2 week evaluation with project plan
SULFO-TAG & Biotin Conjugation	Included	With documented label incorporation (A455)
Assay Optimization	Included	Antibody concentrations, incubation, MRD, matrix
Fit-for-Purpose Qualification	Included	Precision, linearity, recovery, selectivity
Methods Documentation	Included	Full development report with QC criteria

For Research Use Only. All development uses manufacturer-validated MSD reagents and documented protocols.

How to Get Started

Submit Target

Provide target, available antibodies, and intended use context. We'll assess feasibility within 1–2 weeks.

Development

Antibody screening, SULFO-TAG/biotin conjugation, assay optimization with milestone updates.

Receive Qualified Assay

Receive your custom MSD assay with complete development documentation and QC performance data.

Custom Assay Development FAQ

What is the difference between R-PLEX and U-PLEX?

U-PLEX configures custom panels from existing, qualified, linker-coupled antibodies in the MSD catalog — it's a configuration exercise, not development. Timeline: days. R-PLEX develops new MSD assays for targets not in any catalog — full de novo development including antibody screening, SULFO-TAG conjugation, plate spotting optimization, and qualification. Timeline: 8–16 weeks. If your target is in the U-PLEX catalog, use U-PLEX. If it's not, R-PLEX is the path. R-PLEX is MSD's product name for de novo assay creation, distinct from U-PLEX configuration.

Can I provide my own antibodies for custom development or ELISA conversion?

Yes. If you have validated antibody pairs from existing ELISA or other immunoassay platforms, our ELISA-to-ECL conversion service transfers them to the MSD format — capture antibody immobilized on MULTI-ARRAY plates (using MSD ELISA Conversion Packs), detection antibody conjugated with SULFO-TAG. This preserves your reagent investment while gaining ECL's sensitivity (10–100× improvement), wider dynamic range, and multiplex compatibility. For R-PLEX development where antibody pairs are not yet identified, we can screen candidate antibodies or work with recombinant target protein to identify suitable pairs. Contact us with your antibody specifications for technical feasibility assessment.

How is SULFO-TAG label incorporation measured and why does it matter?

SULFO-TAG label incorporation is measured by absorbance at 455 nm — not at 280 nm, because SULFO-TAG absorbs significantly at 280 nm and interferes with standard A280 protein measurements. Protein concentration of the conjugate is determined by BCA assay with IgG standards. For biotin label incorporation on the capture antibody, the goal is typically 2–4 biotin labels per antibody — sufficient for stable streptavidin-plate immobilization without steric hindrance of antigen binding. For SULFO-TAG on the detection antibody, we optimize the challenge ratio (typically starting at 12:1 SULFO-TAG:antibody) to maximize signal without causing antibody aggregation or non-specific binding. These conjugation quality metrics are documented in the development report.

What is the typical development timeline and cost structure?

R-PLEX development follows a milestone-based structure: Phase 1 — Feasibility Assessment: 1–2 weeks, complimentary. Phase 2 — Antibody Screening & Conjugation: 4–8 weeks depending on whether suitable antibodies are already available or require screening. Phase 3 — Optimization & Qualification: 3–6 weeks including precision, linearity, recovery, and selectivity testing in relevant matrix. Total: 8–16 weeks from project initiation to qualified assay, target-dependent. ELISA conversion is typically faster (4–8 weeks) since antibody pairs are already validated. Cost is milestone-based — you commit to each phase after reviewing the previous phase's results. Contact us with your target details for a project-specific timeline and cost estimate.

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