The compass and the map: Navigating biomarker strategies for novel modalities

Key takeaways

Redefining pharmacokinetics: For cell and gene therapies, "dose" is a dynamic concept; biomarkers must track cellular expansion, persistence, and exhaustion (via flow cytometry/qPCR) rather than just simple serum concentration.
The theranostic loop: Radiopharmaceuticals are successfully collapsing the diagnostic-therapeutic divide by using imaging agents (like ⁶⁸Ga-PSMA) as predictive biomarkers for therapeutic isotopes (like ¹⁷⁷Lu).
Liquid becomes solid: Circulating tumor DNA (ctDNA) is moving from a research tool to a potential regulatory surrogate endpoint, offering a faster readout for efficacy than traditional radiographic progression in solid tumors.
Context is king: As FDA guidance evolves, the focus is shifting towards "Context of Use" (COU)—ensuring a biomarker is validated specifically for its role (e.g., patient selection vs. monitoring) rather than just its biological plausibility.

In the small-molecule era, the biomarker equation was often elegantly simple: Is the target present? Is the drug binding to it? (Think HER2+ status and Herceptin). But as the industry pivots to "living drugs"—CAR-T cells, gene therapies, and bispecific antibodies—that static map is no longer sufficient.

For these emerging modalities, the drug is not just a molecule; it is an agent that expands, contracts, exhausts, and interacts dynamically with the host immune system. Consequently, the biomarker strategy must evolve from a static snapshot to a high-fidelity motion picture. We are no longer just looking for the target; we are tracking the journey.

"Living drugs" require dynamic tracking

For an aspirin, pharmacokinetics (PK) is straightforward: Cmax (peak concentration) and half-life. For an autologous CAR-T therapy, PK is a biological event. The "drug" proliferates inside the patient, often peaking days or weeks after infusion.

This has forced translational teams to adopt cellular kinetics as the new PK. The industry is coalescing around quantitative PCR (qPCR) and flow cytometry to measure the expansion and persistence of CAR-T cells in peripheral blood [1]. However, presence does not equal potency. A persisting T-cell that is "exhausted" (expressing high levels of PD-1 or TIM-3) offers little therapeutic value.

Therefore, the next generation of biomarkers for cell therapy is moving beyond simple enumeration. We are seeing a shift toward functional biomarkers—tracking cytokine release signatures (like IL-6 and IFN-γ) not just for safety (CRS prediction) but as correlates of durable efficacy.

The theranostic promise: seeing what you treat

Nowhere is the marriage of biomarker and therapy more intimate than in radiopharmaceuticals. This field has pioneered the "theranostic" concept, where a diagnostic isotope (the scout) pairs with a therapeutic isotope (the soldier).

The commercial success of Pluvicto (¹⁷⁷Lu-PSMA-617) in prostate cancer was built on this foundation. The biomarker here is not a blood test, but a PET scan using a companion diagnostic (⁶⁸Ga-PSMA-11) [2]. This allows for a binary "go/no-go" treatment decision based on actual tumor uptake.

However, challenges remain. As noted in recent reviews of bispecific radioligands, heterogeneity is the enemy. A patient may have "PSMA-positive" disease on a scan, but micro-metastases might express the target at levels too low for the therapeutic isotope to deliver a cytotoxic dose. This is driving research into dual-target radioligands and the use of dosimetry as a predictive biomarker to individualize administered activity [3].

Liquid biopsy: the race for a new endpoint

Perhaps the most significant regulatory shift is occurring in liquid biopsy. For decades, "Overall Survival" (OS) and "Progression-Free Survival" (PFS) have been the ironclad endpoints for oncology approvals. But these take years to read out.

Circulating tumor DNA (ctDNA) is poised to disrupt this timeline. Recent consortia-led studies, such as those by Friends of Cancer Research, have demonstrated that a reduction in ctDNA levels (molecular response) correlates strongly with long-term survival in non-small cell lung cancer (NSCLC) [4].

The FDA is cautiously receptive. The agency's draft guidance on using ctDNA as an early endpoint signals a willingness to accept "molecular clearance" as a surrogate endpoint for accelerated approval, particularly in the neoadjuvant setting where radiographic imaging is often equivocal.

Feature	Traditional Biomarker Strategy	Novel Modality Strategy
Primary Metric	Static expression (e.g., IHC +/++)	Dynamic kinetics (e.g., expansion/decay)
Sample Source	Archived tissue biopsy	Serial liquid biopsy (ctDNA) or Imaging
PK Measure	Serum concentration ($C_{max}$)	Vector copy number / Cellular phenotype
Regulatory Role	Patient selection (Companion Dx)	Surrogate endpoint for efficacy (monitoring)

Navigating the regulatory nebula

The technical ability to measure a biomarker often outpaces the regulatory understanding of what that measurement means. The FDA's recent draft guidance on Cell and Gene Therapy Clinical Trials emphasizes that while innovative endpoints are welcome, they must be "reliable and clinically meaningful" [5].

For developers, this means the biomarker strategy cannot be an afterthought. It must be woven into the clinical development plan from Phase 1. Is your ctDNA assay sensitive enough to detect Minimal Residual Disease (MRD)? Is your imaging agent widely available enough to support a global Phase 3 trial?

In the age of emerging modalities, the drug may be the engine, but the biomarker is the steering wheel. Without it, you are moving fast, but you might just be driving in circles.