Phase 1 Trials in Oncology: Unique Challenges and Considerations

Introduction

Phase 1 clinical trials in oncology differ significantly from those in other therapeutic areas. Unlike traditional Phase 1 studies conducted in healthy volunteers, oncology trials are usually initiated directly in patients with advanced cancers. These trials aim not only to assess safety and pharmacokinetics (PK), but also to explore early signs of efficacy, dose-response, and biomarker correlations. With the rise of immunotherapies, targeted agents, and biologics, designing and executing early-phase oncology trials has become more complex and strategically critical to success in later phases.

Why Oncology Phase 1 Trials Are Different

Several unique characteristics distinguish oncology Phase 1 trials:

• No healthy volunteers: Drugs are often cytotoxic or immunomodulatory, making it unethical to expose healthy subjects
• Therapeutic intent: Even early trials aim to provide some benefit to participants with limited treatment options
• Tumor burden and heterogeneity: Impact pharmacodynamics and biomarker interpretation
• Endpoints often include efficacy measures: Objective response rate (ORR), disease control rate (DCR), etc.

Key Objectives of Oncology Phase 1 Studies

• Determine the maximum tolerated dose (MTD)
• Establish the recommended Phase 2 dose (RP2D)
• Evaluate dose-limiting toxicities (DLTs) and cumulative safety profile
• Assess PK and pharmacodynamic (PD) markers
• Explore preliminary signs of anti-tumor activity

Design Approaches in Oncology Phase 1

1. Traditional 3+3 Dose Escalation

• Cohorts of 3–6 patients receive escalating doses
• If 0/3 or 1/6 experience DLT → escalate
• If ≥2/3 or ≥2/6 experience DLT → stop

Limitations:

• Slow progression through doses
• Poor estimation of true MTD
• No statistical modeling

2. Model-Based Designs (e.g., CRM, BLRM)

• Continual Reassessment Method (CRM): Uses Bayesian probability to determine next dose
• Bayesian Logistic Regression Model (BLRM): Incorporates prior knowledge and observed DLTs
• Allows more efficient escalation and better MTD estimation

3. Accelerated Titration and Intra-Patient Escalation

• Start with low doses in single patients
• Quick escalation if minimal toxicity observed
• Helps reduce number of patients exposed to subtherapeutic doses

Patient Selection and Recruitment

Eligibility Criteria

• Patients with advanced, refractory cancer
• Typically no standard of care remaining
• Good performance status (ECOG 0–1)

Challenges:

• Limited population size for rare cancers
• Competing trials at academic sites
• Ethical complexities of enrolling patients in dose-finding studies

Endpoints Beyond Safety

Although safety and tolerability are primary, oncology Phase 1 studies often incorporate early efficacy and translational endpoints:

• Tumor response (per RECIST/iRECIST criteria)
• Biomarker expression changes
• Immune activation profiles (e.g., T-cell infiltration, cytokine levels)
• Duration of response in expansion cohorts

Dose-Limiting Toxicities (DLTs) in Oncology

DLTs are predefined treatment-related adverse events that guide dose escalation and determination of MTD. They typically occur in the first cycle (often 21 or 28 days).

Common Oncology DLTs:

• Grade 3–4 neutropenia, thrombocytopenia
• Grade ≥3 non-hematologic toxicities (e.g., diarrhea, mucositis)
• Immune-related toxicities for checkpoint inhibitors

Recommended Phase 2 Dose (RP2D)

In some modern oncology trials, RP2D is selected based on tolerability, pharmacokinetics, pharmacodynamics, and biomarker data—not just MTD. This is especially relevant for:

• Immunotherapies
• Targeted agents with wide therapeutic index
• Biologics with flat dose–toxicity relationships

Expansion Cohorts in Phase 1

To further explore safety, efficacy, and biomarkers at the RP2D, expansion cohorts are often built into Phase 1 protocols.

Benefits:

• Collect more robust safety and preliminary efficacy data
• Evaluate activity in biomarker-defined subgroups
• Accelerate development toward pivotal trials

Biomarkers and Translational Research

Modern oncology trials integrate extensive translational endpoints:

• Tumor biopsies (baseline and post-treatment)
• Circulating tumor DNA (ctDNA)
• PD-L1 expression, TMB, MSI status
• Pharmacodynamic markers like Ki-67 or cleaved caspase-3

Ethical and Operational Considerations

• Clear communication about therapeutic uncertainty and risk
• Supportive care protocols for anticipated toxicities
• Independent safety monitoring and early stopping rules
• Early engagement with Institutional Review Boards (IRBs)/Ethics Committees

Regulatory Perspective

• FDA: Encourages integrated designs with expansion cohorts under a single IND
• EMA: Recommends seamless design justification and biomarker validation
• CDSCO: Requires scientific rationale for dose selection and DLT definitions

Best Practices

• Use adaptive dose escalation strategies where feasible
• Define RP2D based on integrated PK, PD, and efficacy data
• Ensure biomarker samples are collected and analyzed in real time
• Involve multidisciplinary teams (clinical, translational, statistical) in protocol design
• Plan for early scalability from Phase 1 to Phase 2 using modular design