Chaperone-Like Therapy with Tetrahydrobiopterin in Clinical Trials for Phenylketonuria: Is Genotype a Predictor of Response?

Abstract

Prospectively enrolled phenylketonuria patients (n ¼ 485) participated in an international Phase II clinical trial to identify the prevalence of a therapeutic response to daily doses of sapropterin dihydrochloride (sapropterin, KUVAN ® ). Responsive patients were then enrolled in two subsequent Phase III clinical trials to examine safety, ability to reduce blood Phenylalanine levels, dosage (5-20 mg/kg/day) and response, and bioavailability of sapropterin. We combined phenotypic findings in the Phase II and III clinical trials to classify study-related responsiveness associated with specific alleles and genotypes identified in the patients. We found that 17% of patients showed a response to sapropterin. The patients harbored 245 different genotypes derived from 122 different alleles, among which ten alleles were newly discovered. Only 16.3% of the genotypes clearly conferred a sapropterin-responsive phenotype. Among the different PAH alleles, only 5% conferred a responsive phenotype. The responsive alleles were largely but not solely missense mutations known to or likely to cause misfolding of the PAH subunit. However, the metabolic response was not robustly predictable from the PAH genotypes, based on the study design adopted for these clinical trials, and accordingly it seems prudent to test each person for this phenotype with a standardized protocol.

Assessment of tetrahydrobiopterin (BH4)-responsiveness and spontaneous phenylalanine reduction in a phenylalanine hydroxylase deficiency population

“…A negative inter-allelic complementation could have a role, preventing better response [51]. In addition, IVS8 − 7 mutation was shown not to be BH 4 responsive in a functionally hemizygous patient, which is in accordance with a single previous report [48]. The BH 4 responsiveness in splice-site mutations could be explained by the existence of residual normal splicing and/or transcripts with a residual enzyme function in those cases [52].…”

“…Their effect depended on the other mutations in the genotype as reported previously [12,27,35,36,38,39,44]. The p.L48S mutation showed a response and slow-response in combination with IVS4 − 5 C>G mutation; unlike in previous reports no response was observed in combination with p.S349P which is a null mutation [48,49]. The p.R158Q mutation was responsive in all four cases with p.E390G, but as previously suggested not in functionally hemizygous patients (combination with p.R408W or possibly with p.R157S) [12,38,39]; furthermore, a slow-response was observed in combination with IVS10 − 3 C>T. The p.R261Q mutation is one of the very few responsive mutations located in the cofactor binding domain [35]; however, the response was observed only in the combination with p.E390G, but not in a combination with a null mutations (EX5del955 and p.R408W), which confirmed most previous reports [36,38].…”

Tetrahydrobiopterin, its Mode of Action on Phenylalanine Hydroxylase, and Importance of Genotypes for Pharmacological Therapy of Phenylketonuria

“…The outcome of the BH 4 test was correlated with the single allele variation or with the genotype [Dobrowolski et al., ; Karacic et al., ; Quirk et al., ; Sarkissian et al., ; Zurflüh et al., ] and for a number genotypes this was found to be a useful complementary tool to BH 4 challenge (Table ). Dobrowolski et al.…”

Genotype‐phenotype correlations and BH4 estimated responsiveness in patients with phenylketonuria from Rio de Janeiro, Southeast Brazil

“…Sarkissian et al. () reported p.V388M as an unresponsive mutation, whereas in the BIOPKU database, of the 14 homozygous patients for the p.V388M mutation tested for BH 4 responsiveness, eight (57.14%) were responsive and two (14.29%) were slow responders. Zurfluh et al.…”

“…Sarkissian et al. () analyzed the genotypes of patients that participated in the clinical trials of sapropterin, a synthetic pharmacological form of BH 4 (6 R ‐L‐ erythro ‐5,6,7,8 tetrahydrobiopterin dihydrochloride). There are some inconsistencies between these authors’ findings and those reported at the BIOPKU database and by Zurfluh et al.…”