Human Equivalent Dose (HED) Calculator

BSA-Based Allometric Scaling

When translating doses from animal studies to humans, a simple milligram-per-kilogram (mg/kg) conversion is insufficient. Smaller animals have higher metabolic rates relative to their body weight, which means they typically clear drugs faster and require higher mg/kg doses to achieve equivalent exposure.

Body surface area (BSA) scaling accounts for these metabolic differences. The relationship between body weight and BSA follows an allometric power law, and BSA correlates more closely with many physiological parameters relevant to drug disposition, including cardiac output, blood volume, renal function, and caloric expenditure.

The FDA recommends BSA-based dose conversion as the default method for estimating human equivalent doses from animal data, particularly for first-in-human dose selection.

The Km Factor Method

The Km factor is a species-specific constant that represents the ratio of body weight (kg) to body surface area (m²). The human equivalent dose is calculated by:

HED (mg/kg) = Animal Dose (mg/kg) x (Animal Km / Human Km)

Alternative Allometric Form

An alternative way to express the interspecies dose conversion uses the body weight ratio raised to the 0.33 power (approximately one-third):

HED = Animal Dose x (W_animal / W_human)^0.33

This form derives from the same underlying allometric relationship (BSA scales approximately as body weight to the 0.67 power). The Km factor method is generally preferred because the FDA has published standardized Km values, reducing variability in calculations.

Important Caveats

While BSA scaling is the standard starting point, several factors can make the actual human dose significantly different from the calculated HED:

• Biologics and large molecules: Peptides and proteins may not follow BSA scaling as well as small molecules. For biological products, the FDA recommends using mg/kg scaling rather than BSA normalization in some cases.
• Nonlinear pharmacokinetics: If a compound exhibits saturation kinetics, dose-dependent absorption, or nonlinear clearance, simple allometric scaling may not accurately predict human exposure.
• Species-specific metabolism: Some compounds are metabolized by enzymes that differ significantly across species in activity, expression, or substrate specificity.
• Route of administration: Bioavailability can vary substantially between species for the same route of administration.
• Target receptor differences: Binding affinity for the drug target may differ across species, affecting both efficacy and toxicity thresholds.

Safety Factors in First-in-Human Dosing

When selecting a starting dose for first-in-human clinical trials, the HED derived from the no-observed-adverse-effect level (NOAEL) in the most sensitive and relevant animal species is divided by a safety factor. The standard approach recommended by the FDA is:

1. Determine the NOAEL from preclinical toxicology studies.
2. Convert the NOAEL to a human equivalent dose using BSA scaling (Km method).
3. Apply a safety factor (typically 10x) to arrive at the maximum recommended starting dose (MRSD).

The standard safety factor of 10 accounts for interspecies differences in drug sensitivity that are not captured by allometric scaling, as well as variability within the human population. In some circumstances, a larger safety factor may be warranted.

References

• FDA Guidance for Industry: Estimating the Maximum Safe Starting Dose in Initial Clinical Trials for Therapeutics in Adult Healthy Volunteers (July 2005).
• Reagan-Shaw S, Nihal M, Ahmad N. Dose translation from animal to human studies revisited. FASEB J. 2008;22(3):659-661. doi:10.1096/fj.07-9574LSF
• Nair AB, Jacob S. A simple practice guide for dose conversion between animals and human. J Basic Clin Pharm. 2016;7(2):27-31. doi:10.4103/0976-0105.177703