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4 minute read
In antibody–drug conjugate (ADC) development, knowing where your drug goes and whether it’s doing what you designed it to do can make the difference between success and costly setbacks. Radiochemistry offers a powerful way to generate that insight, using radioactive isotopes to “tag” antibodies, payloads, or both so their journey through the body can be tracked with precision.
In this guide, we’ll break down the basics of radiochemistry for ADC teams, explain key concepts in radioisotope selection, and share practical tips to avoid common pitfalls.
What Is Radioactivity and Why It Matters for ADCs
Radioactivity is the process by which an unstable atomic nucleus releases energy (decay) to become more stable. For ADC developers, understanding these fundamentals is critical:
- Half-life (t½): The time it takes for half of the atoms to decay. This must align with your ADC’s pharmacokinetics (PK) so the isotope’s signal lasts long enough to track distribution and clearance.
- Decay type: Determines the detection method. For example, PET (Positron Emission Tomography) uses isotopes that emit positrons, which allows scientists to create very detailed, 3D images of how a tracer moves and accumulates in the body. SPECT (Single Photon Emission Computed Tomography) uses isotopes that emit gamma rays, producing images that show biological activity and how a drug or tracer behaves over time.
- Specific activity: Radioactivity per unit mass; higher values mean you can label with very little isotope, minimizing interference with binding or PK.
Decay Modes and Detection Methods
Different isotopes release different types of radiation, which affects how they’re used:
- Alpha (α): Heavy, short-range particles, mainly for therapeutic applications (e.g., Ac-225).
- Beta minus (β–): Electrons; common in therapeutic isotopes like Lu-177 and I-131.
- Beta plus (β+) / positrons: Produce PET photons for high-resolution imaging (e.g., Zr-89, Cu-64).
- Gamma (γ): Photons detected in SPECT imaging (e.g., In-111).
Tip: Match decay type to your goal — positron emitters for imaging, beta/alpha for therapy, gamma for SPECT tracking.
Key Concepts in Isotope Selection for ADCs
- Match half-life to PK: Zr-89 for long-lived antibodies; In-111 for mid-timescale studies; I-131/Lu-177 for longer courses or therapy-linked readouts. Generally, the payload should be radiolabeled with short half-life isotopes.
- Preserve function: Choose high-specific-activity materials and gentle conjugation chemistry.
- Regulatory fit & supply: Pick isotopes with robust supply chains and established handling/documentation.
- Imaging vs therapy: Imaging isotopes maximize detectability; therapeutic isotopes are chosen for their cytotoxic radiation.
Labeling Strategies for ADCs
Direct labeling (iodination)
- Attaches iodine isotopes (I-123/124/125/131) directly to tyrosines / histidines in the antibody.
- Fast and efficient but in vivo radio deiodination can occur. Residualizing tags may be used to avoid in vivo radio deiodination
Indirect labeling (chelation)
- Uses a bifunctional chelator (e.g., DFO for Zr-89; DOTA for Lu-177) conjugated to the antibody before loading the isotope.
- Offers higher in vivo stability; chelator choice depends on isotope chemistry.
- However, although not usual, the chelators could change the PK and/or biological activity of the antibody.
Payload labeling
- Isotope is attached to the cytotoxic payload to monitor release and clearance.
- Can be combined with antibody labeling (dual-label) to differentiate intact ADC from free payload.
Common Pitfalls and How to Avoid Them
- Label instability: Choose isotope–chelator pairs with proven in vivo stability.
- Biological alteration: Avoid harsh labeling conditions that can impair binding or PK.
- PK mismatch: Don’t use a half-life that’s too short to capture late-phase distribution or clearance.
Quick Reference: Isotopes for ADC Applications
| Isotope | Half-life | Emission Type | Imaging Modality / Typical Use | Typical Tracking Window | Notes |
|---|---|---|---|---|---|
| Zr-89 | 78.4 h (~3.3 d) | β+ (positron) | PET imaging – ideal for long-lived antibodies | Up to 7–10 days | Matches antibody PK; provides high-resolution PET images for extended studies |
| Lu-177 | 6.65 d | β– (beta) | Therapeutic payload; can also support imaging | Days to 1–2 weeks | Dual-use radionuclide (therapy + imaging); strong track record in radiopharma |
| I-131 | 8.02 d | β–, γ (beta and gamma) | Therapy and imaging for antibody/payload ADME | 1–2 weeks | Widely used in radioimmunotherapy; dual imaging/therapy capacity |
| In-111 | 2.8 d | γ (gamma) | SPECT imaging – mid-timescale biodistribution | 1–5 days typical (up to ~10 with cut-and-count) | Best suited for 1–5 day studies; imaging resolution optimal in shorter window |
| Ac-225 | ~10 d | α (alpha) | Targeted alpha therapy | Days to weeks (therapy-focused) | Very high linear energy transfer (LET); highly cytotoxic, therapeutic only |
| Cu-67 | 2.6 d | β– (beta) | Therapy; theranostic partner with Cu-64 | Several days | Can be paired with Cu-64 PET (same chemistry) for theranostic workflows |
Using Radiotracers in PDX and CDX Models
Radiotracer studies can be performed in a range of preclinical models, but model selection directly affects how translatable your data will be. The two most common approaches for ADC biodistribution are patient-derived xenograft (PDX) models and cell line-derived xenograft (CDX) models.
PDX Models
- What they are: Tumors from actual patients implanted into immunodeficient mice, retaining original histology and molecular characteristics.
- Strengths: Closely mimic human tumor biology, heterogeneity, and target expression; often more predictive of clinical outcomes.
- Weaknesses: More variable growth rates, higher cost, and sometimes limited availability for rare targets.
CDX Models
- What they are: Tumors grown from established cancer cell lines implanted into mice.
- Strengths: Easier to grow, faster to establish, and highly reproducible; good for early proof-of-concept and method development.
- Weaknesses: Less heterogeneity and may not fully recapitulate the target expression or microenvironment seen in patients.
Choosing the Right Model
For ADC radiotracer studies, CDX models can be a cost-effective starting point to validate isotope choice and labeling chemistry, while PDX models are best for confirming biodistribution and target engagement in a clinically relevant setting before moving to the clinic. Many developers use both — starting in CDX for feasibility and scaling into PDX for translational validation.
Study Design Checklist for ADC Radiolabeling
- Define your primary question: target engagement, linker stability, payload distribution, or therapy evaluation.
- Select isotope(s) to match PK and imaging/therapy needs.
- Choose labeling chemistry that preserves ADC function.
- Plan imaging modality and sampling timepoints to capture both early and late phases.
- Combine imaging with ex vivo biodistribution for quantitative confirmation.
- Include mass-dose escalation to determine receptor saturation.
