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GOOD REGULATORY PRACTICES · 04

Good Regulatory Practices #4: Building the Bridge from Animal Studies to First-in-Human Trials

Connect nonclinical evidence, species selection and dose escalation to a safe first-in-human plan.

Dr. Leona Saunders, IRSG · 8 min read

Introduction

The most important decision you'll make in early drug development happens before your first patient ever receives a dose: choosing your starting dose and dose escalation plan for Phase 1.

Get it right, and you progress safely and efficiently through early development. Get it wrong, and you either expose patients to unacceptable risk or waste months in overly conservative escalation that delays your program. From large to small pharma no one is exempt from getting this completely wrong.

This decision is based on animal studies that must convince FDA's pharmacology/toxicology reviewers that you understand your drug's safety profile well enough to predict human response. It's informed by a PK/PD plan that FDA's clinical pharmacology reviewers will scrutinize to ensure you're learning what you need to learn while keeping patients safe.

The bridge from animal studies to first-in-human (FIH) trials is where nonclinical development meets clinical. And it's an area where FDA has clear expectations and increasingly, changing guidances that reflect the advancement in science.

FDA's Core Question: Do Your Animal Studies Support Human Dosing?

When FDA's pharmacology and toxicology reviewers assess your IND, they're asking a simple question: Do your animal studies provide sufficient safety support for the starting dose and support the schedule in your proposed clinical trials?

"Sufficient" doesn't mean perfect. It means you've done the studies needed to make a scientifically sound prediction about where to start in humans, what toxicities to watch for, and how to escalate safely.

In Simple Terms "Sufficient" Actually Means

Species selection: You've chosen appropriate animal species based on metabolic similarity, target expression, or pharmacologic activity, not just convenience.

Study design: Your toxicology studies cover the duration, route, and dosing schedule relevant to your clinical plan.

Safety margins: You can calculate a reasonable starting dose with adequate safety margins based on animal data.

Toxicity characterization: You know what organs are at risk, what the early warning signs are, and how to monitor for them in humans.

Reversibility: You understand whether toxicities are reversible, what drives them, and what that means for human dosing.

If FDA doesn't believe your animal package adequately addresses these points, you'll get a clinical hold before your first patient is ever enrolled.

Species Selection: Relevance Trumps Tradition

One of the most important decisions in your nonclinical program is which animal species to use. The wrong choice can invalidate your entire safety package.

The Relevance Question

What FDA wants to know: Is the chosen species actually predictive of human response?

This depends on several factors:

Metabolic similarity: Does the species metabolize your drug similarly to humans? Are the major metabolites the same?

Target expression: For biologics, does the species express the human target? Does the drug bind with similar affinity?

Pharmacologic activity: Is the drug pharmacologically active in the species? Inactive drugs don't produce toxicity relevant to human risk.

Historical precedent: What has been used successfully for similar compounds? What has failed?

The Evolving Landscape: Reducing NHP Use for Monoclonal Antibodies

FDA's recent draft guidance (released December 2025) represents a significant shift: for monoclonal antibodies (mAbs) that bind to highly conserved targets across species, sponsors may be able to reduce or eliminate NHP testing.

Key principles from the guidance:

  • If your mAb binds to the target in a relevant small animal species with comparable affinity and produces similar pharmacologic effects, you may not need NHPs
  • In vitro tissue cross-reactivity studies can help identify appropriate species
  • For targets with broad tissue expression and high conservation, rodent studies may suffice
  • NHPs should be used when they're the only relevant species, not by default

What this means for sponsors:

This isn't permission to skip safety testing; it's an opportunity to use more appropriate models. If you can demonstrate that your mAb is pharmacologically active in rats or cynomolgus monkeys, and that the toxicity profile in that species is relevant to humans, you may use that species instead of defaulting to NHPs.

Your move: For mAb programs, conduct thorough tissue cross-reactivity studies early. Characterize binding affinity and pharmacologic activity across species. Discuss species selection in your Pre-IND meeting, FDA wants to see your scientific rationale, not just tradition.

Practical implications:

  • Reduced animal use aligns with 3Rs principles (Replace, Reduce, Refine)
  • Lower costs and shorter timelines if you can use rodents instead of NHPs
  • Requires more rigorous upfront characterization to justify species choice
  • Still emerging, expect FDA to evaluate case-by-case until precedent builds

Starting Dose Selection: The Safety Margin Calculation

Your starting dose in humans must be based on animal safety data with appropriate margins to account for uncertainty.

The Standard Approach

Step 1: Identify the No Observed Adverse Effect Level (NOAEL)

From your most sensitive relevant species (the species showing toxicity at the lowest dose), identify the highest dose that produced no adverse effects.

Step 2: Calculate Human Equivalent Dose (HED)

Convert animal doses to human equivalent using body surface area or pharmacokinetic adjustments:

  • For small molecules: Typically use body surface area (BSA) normalization
  • For biologics: May use body weight or PK-based approaches depending on the molecule

Step 3: Apply Safety Factor

Divide the HED by an appropriate safety factor (typically 10-fold for most drugs) to establish a starting dose that provides adequate margin for individual variability and uncertainty.

When Standard Approaches Don't Apply

Highly toxic compounds: May need larger safety factors (50- or 100-fold)

Pediatric first-in-human: Additional considerations for vulnerable populations

Target-mediated toxicity: When the target itself is the concern, animal data may not fully predict human response

Immunomodulators: Cytokine release and immune-related toxicities may not scale linearly

What FDA Expects to See

  • Clear identification of NOAEL in each species tested
  • Transparent dose conversion methodology
  • Justified safety factor with scientific rationale
  • Discussion of any uncertainties or special considerations
  • Alternative approaches if standard methods don't apply (with FDA agreement)

Your move: Don't just calculate a starting dose, explain your reasoning. FDA reviewers want to see your thought process, not just the final number.

Dose Escalation Strategy: Balancing Speed and Safety

Your dose escalation plan must balance efficient progress against patient safety.

Standard 3+3 Design

  • Start with 3 patients at the starting dose
  • If no dose-limiting toxicities (DLTs), escalate to next dose level
  • If 1 DLT in 3 patients, expand to 6 patients at that dose level
  • If ≥2 DLTs, that dose exceeds the maximum tolerated dose (MTD)

Pros: Conservative, well-understood by FDA Cons: Can be slow, treats many patients at subtherapeutic doses

Accelerated Titration Designs

  • Faster escalation (e.g., 100% dose increases) in initial cohorts
  • Single patients or smaller cohorts until toxicity is seen
  • Transition to standard 3+3 when predefined criteria are met

Pros: Faster, exposes fewer patients to subtherapeutic doses Cons: Requires FDA agreement, must have strong preclinical justification

Model-Based Designs (CRM, EWOC)

  • Use Bayesian models to estimate MTD based on accumulating data
  • Can be more efficient than algorithm-based designs
  • Increasingly accepted by FDA with proper safeguards

Pros: Statistically more efficient, flexible Cons: More complex, requires modeling expertise, FDA may want Pre-IND discussion

Maximum Dose and Duration

You need a plan for when to stop escalating, even if you haven't seen DLTs:

Biologic ceiling: Target saturation, receptor occupancy plateaus Exposure ceiling: PK shows exposures well above animal NOAEL equivalents Practical ceiling: Formulation or dosing logistics become unreasonable Predefined maximum: Based on regulatory or scientific considerations

Your move: Don't just escalate until you hit toxicity. Have scientifically justified stopping rules based on PK/PD data.

Bringing It All Together: The Integrated Safety Plan

Your IND should present an integrated story connecting nonclinical findings, species selection, starting dose calculation, monitoring plan, and PK/PD strategy.

What FDA reviewers want to see:

  • Clear rationale: Why these species? Why this starting dose? Why this escalation plan?
  • Transparent data: Don't hide concerning findings, explain them and show how you'll manage risk
  • Comprehensive monitoring: You've thought through what could go wrong and have a plan to detect it
  • Flexibility with justification: If you're proposing something non-standard, explain why it's appropriate for your drug
  • Learning objectives: What will you learn in Phase 1 that informs Phase 2?

When to Engage Nonclinical and Clinical Pharmacology Expertise

Consider bringing in specialized regulatory support when:

  • Novel modality: First-in-class mechanism, new therapeutic modality, limited regulatory precedent
  • Complex PK: Nonlinear kinetics, target-mediated clearance, unexpected metabolism
  • Significant toxicity findings: Serious or unexpected findings in animals that need strategic interpretation
  • Species selection questions: Uncertain which species is most relevant, considering alternatives to NHPs
  • Special population considerations: Pediatrics, pregnant women, or other vulnerable populations
  • Accelerated development pathways: Breakthrough therapy or expedited programs requiring more sophisticated dose-finding

The Bottom Line

The bridge from animal studies to first-in-human trials is built on scientific rigor, transparent reasoning, and comprehensive safety planning.

FDA's reviewers, pharmacology/toxicology experts and clinical pharmacologists, are evaluating whether you understand your drug well enough to dose humans safely and learn what you need to learn for later development.

This isn't about checking regulatory boxes. It's about demonstrating that you've done the science necessary to make informed decisions about human exposure. The sponsors who progress efficiently through Phase 1 are the ones who invested in the right nonclinical studies, chose appropriate species, calculated starting doses conservatively but not excessively, and designed monitoring plans based on what their animal data predicted.

And increasingly, as FDA's recent guidance on reducing NHP use for mAbs demonstrates, it's about applying scientific advances to make better decisions, decisions that are more ethical, more cost-effective, and ultimately more predictive of human response.

Start with the science. Build your plan on data. Show your reasoning. FDA will follow your logic if it's sound.

About This Series

Good Regulatory Practices is an educational series designed to share practical regulatory knowledge that helps sponsors navigate drug development more effectively. Topics are drawn from real-world experience and address the strategic questions that make the difference between efficient development and costly delays.

Dr. Saunders is the Founder and CEO of Innovative Regulatory Science Group (IRSG), a regulatory consulting firm with over 25 years of FDA regulatory experience. She also serves as adjunct faculty at Northeastern University, teaching regulatory strategy for product development.

She is the Founder and CEO of tBrexa Bio, a woman- and minority-owned next-generation CDMO specializing in biologics, gene therapies, and AI-enabled manufacturing. Through tBrexa, she integrates regulatory intelligence, CMC strategy, and advanced bioprocessing to support innovators from IND planning to commercial readiness.

Educational planning content, not regulatory advice. Program-specific decisions require qualified quality, manufacturing and regulatory review.