title: START, a double blind, placebo-controlled pharmacogenetic test of responsiveness to sapropterin dihydrochloride in phenylketonuria patients.
abstract: Sapropterin dihydrochloride, a synthetic tetrahydrobiopterin (BH4), works as a chaperone of phenylalanine hydroxylase (PAH) in phenylketonuria (PKU) to facilitate and stabilize folding of PAH into its most active conformation. No standard pharmacogenetic tests exist to identify responsive genotypes. Previous studies have failed to identify genotypes that consistently predict response; they are weakened by varied: 1) doses; 2) response definitions; 3) duration; 4) phenylalanine (PHE) test times during different protein catabolic states; 5) control of dietary PHE. START (sapropterin therapy actual response test) protocol is a double blind, placebo-controlled, 4-week clinical test that obviates the confounders aforementioned. START results were evaluated for response-genotype correlates and trends in molecular characteristics.
  
  
  RESULTS
  Seventy-four patients completed START. Thirty-six patients (48.6%) responded, 55 patients' genotypes are known, 38 unique genotypes are present. Alleles consistently associated with response include Y414C (8/8 patients, 6 genotypes) and I65T (9/9 patients, 6 genotypes). The p.R408W mutation, in which substitution of straight chain arginine with bulky aromatic amine, tryptophan, at the crux of a strategic hinge site activating folding of PAH, amino acid sequence 408, was strongly associated with non-response (21/29 patients non-responsive, 12/17 genotypes non-responsive). Genotypes containing at least one allele with ≥25% residual activity compared to wild type, were strongly associated with response.
  
  
  CONCLUSIONS
  The START protocol provides a rigorous pharmacogenetic test to identify sapropterin responsiveness and genotypes associated with responsiveness and non-responsiveness. Some genotypes were found to be predictive of responsiveness or non-responsiveness, and responsiveness was associated with specific alleles. The START protocol provides a reliable test for sapropterin responsiveness and will continue to improve understanding of how PKU mutations impact PAH protein-folding dynamics and enhance understanding of PKU disease and its management.
authors: University of Minnesota, Fairview, Department of Pediatrics, Minneapolis, MN 55455, USA. jutz1@fairview.org
authors: Jeanine R Jarnes
authors: JR
authors: Utz
authors: Cindy Pham
authors: CP
authors: Lorentz
authors: Dorothy
authors: D
authors: Markowitz
authors: Kyle D
authors: KD
authors: Rudser
authors: Brenda
authors: B
authors: Diethelm-Okita
authors: David
authors: D
authors: Erickson
authors: Chester B
authors: CB
authors: Whitley
mesh_terms: D000328:Adult; D000483:Alleles; D001708:Biopterins / Q000031:analogs & derivatives* / Q000493:pharmacokinetics / Q000494:pharmacology / Q000627:therapeutic use; D002675:Child, Preschool; D004305:Dose-Response Relationship, Drug; D004311:Double-Blind Method; D004789:Enzyme Activation / Q000187:drug effects; D005260:Female; D005838:Genotype; D006801:Humans; D008297:Male; D009154:Mutation; D010649:Phenylalanine / Q000235:genetics / Q000378:metabolism*; D010651:Phenylalanine Hydroxylase / Q000097:blood / Q000235:genetics / Q000378:metabolism*; D010661:Phenylketonurias / Q000188:drug therapy* / Q000378:metabolism
chemical_list: D001708:Biopterins; D010649:Phenylalanine; D010651:Phenylalanine Hydroxylase; C003402:sapropterin