Development of a protocol for newborn screening for disorders of the galactose metabolic pathway

Application of DBS sampling in combination with LC–MS/MS for pharmacokinetic evaluation of a compound with species-specific blood-to-plasma partitioning

Background: Dried blood spot (DBS) sampling in combination with LC–MS/MS has been used increasingly in drug discovery for quantitative analysis to support pharmacokinetic (PK) studies. In this study, we assessed the effect of blood-to-plasma (B:P) partitioning on the bioanalytical performance and PK data acquired by DBS for a compound AMG-1 with species and concentration-dependent B:P ratio. Results: B:P partitioning did not adversely affect bioanalytical performance of DBS for AMG-1. For rat, (B:P ratio of 0.63), PK profiles from DBS and plasma methods were comparable. For dog, concentration-dependence of B:P ratio was observed both in vivo and in vitro. Additional studies demonstrated concentration-dependence of the compound’s unbound fraction in plasma, which may contribute to the concentration-dependence of the B:P ratio. Conclusion: DBS is a promising sampling technique for preclinical pharmacokinetic studies. For compounds with high B:P ratio, caution needs to be applied for data comparison and interpretation between matrices.

Early cataract formation due to galactokinase deficiency: impact of newborn screening

BACKGROUND AND AIMS

Galactokinase (GALK) deficiency is an autosomal recessive disorder causing cataract formation that can be prevented or mitigated by early diagnosis and galactose-restricted diet. The aim of this retrospective study was to explore whether GALK-deficiency meets the criteria for neonatal mass screening programs.

METHODS

From 2000 until 2010, the Screening Laboratory Hannover performed newborn screening in 1,950,927 infants from Germany for galactosemia by measuring galactose-1-phosphate-uridyl-transferase and total galactose concentration (free galactose plus galactose-1-phosphate), including automatic screening for GALK deficiency.

RESULTS

Eleven cases were found with elevated galactose levels accompanied by normal transferase activity. Nine of 11 cases were informative; the diagnosis was established by demonstrating deficient activity of the GALK enzyme in erythrocytes. To our knowledge, screening did not produce any false negative results. All patients were treated with a galactose-restricted diet from the neonatal period or infancy. Three of nine patients suffered from congenital cataracts or eventual development of cataracts, despite normal galactose concentrations in blood.

CONCLUSIONS

Newborn screening for GALK deficiency prevents or at least mitigates cataract formation. As screening for GALK deficiency is technically simple, it seems to be reasonable to include this disorder in routine screening programs by simultaneous determination of transferase activity and quantification of galactose plus galactose-1-phosphate in dried blood spots.

Pharmacokinetic considerations as to when to use dried blood spot sampling

In the past few years there has been a large increase in the reporting of the use of dried blood spots (DBS) in drug development. Most of these reports pertain to the technological improvements that have allowed for drug concentration measurements from microliter volumes of sample, issues concerning method development, and exploration of the technique, into other areas such as measurement of macromolecules and metabolite identification. One area that has received less attention and is the subject of this commentary concerns the pharmacokinetic issues that arise from using DBS as opposed to plasma, the mainstay matrix. Measurements of drug concentrations from either plasma or dbs are almost always the sum of bound and unbound drug, but it is the unbound drug in plasma (plasma water) that is the relevant driver of essentially all pharmacokinetic and pharmacodynamic events. Therefore, the critical assumption made is constancy in fraction unbound for plasma, and additionally for blood, constancy of hematocrit and blood cell affinity. Often these assumptions are reasonable and either matrix suffices, but not always. Then the value of one matrix over the other depends on the magnitude of the blood-to-plasma concentration ratio of drug, its clearance and the cause of the deviation from constancy. Additional considerations are the kinetics of distribution within blood and those arising when the objective is assessment or comparison of bioavailability. Most of these issues can be explored and addressed in vitro prior to the main drug development program.

Galactosemia: screening and diagnosis

Galactose is normally metabolized to glucose through the coordinated activities of three enzymes: galactokinase, galactose-1-phosphate uridyl transferase (GALT), and uridine diphospho-glucose 4-epimerase (epimerase). High concentrations of galactose and their metabolites are toxic to mammals. Hereditary deficiencies of galactokinase and of GALT and perhaps rarely of epimerase cause clinical disorders that can be prevented by early recognition and institution of a galactose-free diet. The genetics of disorders of galactose metabolisms and the methods used currently for their detection are reviewed. Future prospects in the diagnosis of these disorders are discussed.

Results of newborn screening for galactose metabolic disorders

A screening strategy has been used which uses the Paigen and Beutler methods for the determination of galactose and galactose-1-phosphate. A blood spot test for epimerase has also been developed. In the last 10 years, 265,019 samples from newborns have been tested by these methods. Among the 154 screening positives, we have detected seven cases of epimerase-deficient galactosaemia (Type III), seven cases of Duarte/galactosaemia heterozygotes, 48 cases of other various types of heterozygotes, four cases of persistent hypergalactosaemia, three cases of hepatitis and one case of congenital atresia of the bile duct. These results indicate that our screening system has effectively detected the infants with galactose metabolic disorders.

Galactose-1-phosphate uridyl transferase activity associated with age at menopause and reproductive history

Reproductive history was obtained and the activity and electrophoretic pattern of the blood enzyme galactose-1-phosphate uridyl transferase (transferase) was measured in 104 adult Caucasian women, less than 70 years of age, sampled from the general population. Fifteen women were identified as carriers for the Duarte (GtD) or galactosemia (gt) variants of transferase--genes associated with reduced transferase activity compared with normal (Gt+). The mean age at menopause for 8 women with a natural menopause who were GtD/Gt+ or gt/Gt+ was 44.8, significantly younger (P = 0.007) than the mean age of 49.2 reported by 31 naturally postmenopausal subjects with Gt+/Gt+ genotypes and normal transferase activity. Compared with the latter group, women who were Duarte or galactosemia carriers were 13.7 times more likely to have a menopause before age 48 (with 95% confidence limits of 2.0 to 95.5). Six of 13 (46%) married women who were GtD/Gt+ or gt/Gt+ reported more than 2 years' trying to achieve a pregnancy, compared with 11 of 74 (15%) with normal genotypes and activity (P = 0.02). The authors conclude that genetic deficiency of transferase may be associated with infertility and early menopause.

Neonatal screening and an intensive management programme for galactosaemia: early evidence of benefits

A prospective study of children with galactosaemia is being undertaken at the Metabolic Clinic, Royal Children's Hospital, Brisbane. The purpose of the study is to collect biochemical, clinical, dietary, developmental and speech and language data. This paper describes the operation of the multiprofessional management programme and reports results for two groups of children: those who were diagnosed before the introduction of neonatal screening in 1982 (the prescreening group) and those who were identified by screening. The eight children in the prescreening group have shown intellectual development in the low-average to moderately-handicapped range. Most of them have speech and language difficulties. The screening group, all of whom are still in infancy or early childhood, appears to be developing normally, with the exception of one child who is showing problems with speech and language. The early results provide a basis for cautious optimism that neonatal screening and careful management will result in improved outcomes for children with galactosaemia.

Newborn screening for cystic fibrosis using an enzyme linked immunoabsorbent assay (ELISA) technique

Neonatal screening for cystic fibrosis (CF) has been conducted using conventional radioimmunoassay. An alternative immunoassay approach has been developed and applied to screening 35,550 newborns. Seventeen confirmed CF infants were detected by both assays. This monoclonal antibody-based enzyme immunoassay using microtitre plate ELISA technology has proved effective in case finding and offers a number of advantages. A reduced labour component, ease of handling, use of nonradioactive reagents, long reagent shelf-life, greater specificity and a reduction in potential sample handling and transposition errors combine to make this technology appropriate for a large volume neonatal screening laboratory.