National Institute of Child Health and Human Development Workshop on Extrapolation of Non-clinical Models to Pediatric Clinical Studies: Setting the Research Agenda

S Contributors Tracy S. Zoetis, M.S. Amy L. Lavin, Ph.D. (Now with Milestone) Amy L. Lavin, Ph.D. Michael J. Yang Joseph F. Holson, Ph.D. S

Prenatal Models, Ontogeny, and Postnatal Life

Conceptual Roadmap of Embryonic Development Cellular Potency Cellular Differentiation DeSesso, 1997

Gestational Milestones for Mammals B C D E Primitive Early Organogenesis Usual Species Implantation Streak Differentiation Ends Parturition Rat 5-6 8.5 10 15 21-22 Mouse 5 6.5 9 15 19-20 Rabbit 7.5 7.25 9 18 30-32 Hamster 4.5-5 7 8 13 16 Guinea Pig 6 12 14.5 ~29 67-68 Monkey 9 17 21 ~44-45 166 Human 6-7 13 21 ~50-56 266 1In gestational days; day of confirmed mating = gestational day 0 2Letters refer to positions on Figure 4 (Conceptual Roadmap of Embryonic Development) DeSesso, 1997

Ontogeny Recapitulates Phylogeny (von Baer, 1828) General features appear earlier in embryos than do specialized features Embryos of higher animals pass through stages that are similar to those of embryos of lower species

Ontogeny Recapitulates Phylogeny

Why should we be interested in organ system maturation?

Why should we be interested in organ system maturation? It is essential for comparing postnatal toxicity among species

Concept of Physiologic Time

Concept of Physiologic Time Comparisons among Species Comparisons among Developmental Stages

Maturational Data for Various Species Syrian Hamster Guinea Pig Rhesus Monkey Human Mouse Rat Rabbit Gestation (days) 267 16 20 22 32 63 167 Minimal Breeding Age (weeks) 5 6.5 28 32 4 10 728 7 10.5 218 Human to Animal Life Span 1.0 66 44 33 12 17 4.4

Physiologic Time A method for scaling the lifespan of different species so that comparable stages of maturation are congruent, regardless of chronological age

Time to Develop Adult Characteristics 100 Rat Human % Adult Status 5 10 15 20 Age (years)

Relationship Between Extent of Maturation and Birth in Rats and Humans 100% Adult Status = Birth Maturation Human Rat Conception Physiologic Time

Difficulties in a priori Selection of Models for Preclinical Pediatric Toxicity

Relationship Between Development and Phenotypic Diversity Extent of Differentiation Embryonic Period Fetal Period Postnatal Period Degree of Phenotypic Variability Birth Time in Development (Age)

Pulmonary System

Human Fetal and Postnatal Lung Development

Developmental Stages - Morphology Embryonic Stage (Gestational Days 33-37) Pseudoglandular Stage (Gestational Week 13) Canalicular Stage (Gestational Week 20) Saccular Stage (Gestational Week 34) Alveolar Stage (2 weeks of age)

Interspecies Comparison of Alveolar Development Species Onset At Birth Critical Period Completion Human GWK36 Beginning Birth - 2 years 6-8 years Dog ? Beginning 0 - 16 weeks 16 weeks Monkey ? Completed in utero Birth Rat PND4 Not Started 4 - 14 days 3 weeks Rabbit ? Beginning ? 6 weeks Ferret ? Not Started ? 6 months Mouse PND4 Not Started 3 - 14 days 3 weeks Modified from Zoetis, 1999

Conclusions - Lung Pulmonary development occurs in stages that span prenatal and postnatal periods Stages are similar in humans and animals Timing of stages differs between species Species differ with regard to lung maturity at birth

Conclusions - Lung Pulmonary development occurs in stages that span prenatal and postnatal periods Stages are similar in humans and animals Timing of stages differs between species Species differ with regard to lung maturity at birth Timing of alveolar development is the critical factor in species selection for models of the human pediatric lung

Cardiac System

Comparison of Perinatal Status of Cardiac Structure & Function in Humans & Rats Parameter Humans Rats Relative Heart Weight Constant Decreasing Coronary Circulation Major branches established Major branches of coronary at birth arteries develop during first 2 weeks after birth Cardiac Myocytes Increase in number & size Increase in number & size Autonomic Innervation Established & functional Becomes established & at birth matures functionally during T-tubule System Functional at birth Formed during first 2 weeks after birth Heart Rate Decreasing Increasing through first week, decreasing beginning in second week

Conclusions - Heart Key features of perinatal cardiac microstructure and function have not been studied in sufficient depth to assess interspecies concordance. For example, Establishment of autonomic innervation Status of sarcoplasmic reticulum and T-tubule system Stage for development of coronary circulation Ontogeny of heart rate

Conclusions - Heart Because new pharmaceuticals or other chemicals may interfere with various aspects of perinatal heart development, selection of appropriate animal models can only be accomplished by establishing concordance between the developmental stages in animals and pediatric patients.

Current Models and Safety Test Designs Challenges Regarding Current Models and Safety Test Designs

Pediatric Classifications 12 to <16 Years Adolescents 2 to 12 years Children 1 month to 2 years Infants Birth to 1 month Neonates

Non-Human Developmental Classifications 14-26 36-48 20-28 35-60 Adolescent 4-14 6-36 6-20 21-35 Child 2-4 0.5-6 3-6 10-21 Infant <2 <0.5 <3 <10 Neonate Mini-Pig (wks) Primate (mos) Dog (wks) Rat (days) Category

Non-Human Developmental Classifications 14-26 36-48 20-28 35-60 Adolescent 4-14 6-36 6-20 21-35 Child 2-4 0.5-6 3-6 10-21 Infant <2 <0.5 <3 <10 Neonate Mini-Pig (wks) Primate (mos) Dog (wks) Rat (days) Category

Similarity of Age Should Be Measured in Physiologic Time Comparable categories for ages of various animal species are dependent on the anatomical and functional status of individual organs or systems

Conclusions Parallelism exists among species regardless of lifespan Animal models are predictive only if they mimic the human situation that is of interest Anatomical stages, when important State of physiological or functional maturation Concept of similarity in physiologic time Additional measurements and changes to current guidelines could increase our ability to predict postnatal toxicity