Population PK/PD model of homocysteine concentrations after high-dose methotrexate treatment in patients with acute lymphoblastic leukemia.

Elevated homocysteine concentrations have been associated with methotrexate-induced neurotoxicity. Based on methotrexate and homocysteine plasma concentrations of 494 children with acute lymphoblastic leukemia treated with high-dose methotrexate in the TOTAL XV study, a pharmacokinetic/pharmacodynamic (PK/PD) model was built with NONMEM. Several compartment and indirect response models were investigated. The pharmacokinetic disposition of methotrexate was best described by a two-compartment model. Homocysteine concentrations were included by an indirect response model where methotrexate inhibition of the homocysteine elimination rate was described by an E(max) model. The homocysteine baseline level was found to be age-dependent. Simulations revealed that folinate rescue therapy does not affect peak concentrations of homocysteine but leads to a modestly reduced homocysteine exposure. In conclusion, our PK/PD model describes the increase of methotrexate-induced HCY concentrations with satisfactory precision and can be applied to assess the effect of folinate regimens on the HCY concentration-time course.

Identification and quantification of the atypical metabolite ornithine-lactam in human plasma by liquid chromatography-tandem mass spectrometry (LC-MS/MS).

In the late 1970s the atypical metabolite of ornithine, ornithine-lactam, has been observed in urine samples of patients suffering from hyperornithinemia. However, not least due to insufficient analytical methods, until now there are no data available about ornithine-lactam in human plasma. Here, we describe a new method, which is, for the first time, suitable to identify and quantify ornithine-lactam in human EDTA-plasma. The method was validated according to the requirements of the FDA guidance for bioanalytical method validation. The analytes were extracted on mixed mode cation exchange SPE columns, separated on a silica analytical HPLC column working in the HILIC mode and detected on a tandem mass spectrometer equipped with an ESI ion source. As internal standard newly synthesized stable isotope labeled D(6)-ornithine-lactam was used. The calibration function was linear in the range of 0.1-5 microM. Intra- and inter-day precision and accuracy was better than 14% at all concentration levels. In EDTA-plasma samples from 30 volunteers ornithine-lactam concentrations ranging from 0.136 to 0.653 microM were determined. These concentrations correlated significantly (p<0.001, R(2)=0.784) to those of ornithine in EDTA-plasma.

Methotrexate-associated alterations of the folate and methyl-transfer pathway in the CSF of ALL patients with and without symptoms of neurotoxicity.

BACKGROUND: Severe neurotoxicity has been observed after systemic high-dose and intrathecal methotrexate (MTX) treatment. The role of biochemical MTX-induced alterations of the folate and methyl-transfer pathway in the development of neurotoxic symptoms is not yet fully elucidated. PROCEDURE: MTX, 5-methyltetrahydrofolate, calcium folinate, S-adenosylmethionine, and S-adenosylhomocysteine were measured in the cerebrospinal fluid (CSF) of 29 patients with acute lymphoblastic leukemia (ALL) who were treated with high-dose MTX (5 g/m(2)) followed by calcium folinate rescue (3 x 15 mg/m(2)) and/or intrathecal (8-12 mg) MTX. Two patients developed subacute MTX-associated neurotoxicity. CSF was obtained by lumbal puncture 1-3 weeks after administration of MTX and shortly after the occurrence of neurotoxicity. The analytes were measured using HPLC assays with UV and/or fluorescence detection. RESULTS: In non-toxic patients, CSF concentrations of 5-methyltetrahydrofolate and S-adenosylmethionine were in the normal range 2 weeks after administration of high-dose and intrathecal MTX followed by rescue. In contrast, when these patients received intrathecal MTX without rescue, 5-methyltetrahydrofolate concentrations were significantly decreased 12 days after the first MTX administration. S-adenosylmethionine concentrations were significantly decreased up to 45 days. The two patients suffering from neurotoxicity had decreased levels of 5-methyltetrahydrofolate and S-adenosylmethionine during or following toxicity. S-adenosylhomocysteine was determined in all samples of neurotoxic patients but was below the limit of quantification in most samples of non-toxic patients. Calcium folinate was not detected; MTX was present only in samples obtained during its infusion. CONCLUSION: Intrathecal MTX without folinate rescue as well as MTX-associated neurotoxicity are likely to be associated with specific alterations of the folate and methyl-transfer pathway.

Biochemical and clinical aspects of methotrexate neurotoxicity.

Acute, subacute and chronic neurotoxicity have been observed after the administration of high-dose and/or intrathecal methotrexate (MTX). Acute toxicity is usually transient without permanent damage. Subacute and chronic toxicity are associated with changes in the brain and/or the spinal cord which may be progressive and even lead to coma and death in severe cases. It is believed that MTX can induce direct toxic effects to the CNS by damaging the neuronal tissue. Moreover, MTX interferes with the metabolic pathways of folates, excitatory amino acids, homocysteine, S-adenosylmethionine/S-adenosylhomocysteine, adenosine and biopterins, inducing biochemical alterations which have been associated with neurotoxic symptoms. It has been suggested that acute toxicity is partly mediated by adenosine, whereas homocysteine, S-adenosylmethionine/S-adenosylhomocysteine, excitatory amino acids and biopterins may play an important role in the development of subacute and chronic toxicity. A better understanding of the pathogenesis of MTX neurotoxicity would offer the possibility of developing new therapeutic strategies for its treatment or prevention.

Simultaneous analysis of sulfur-containing excitatory amino acids using micellar electrokinetic chromatography with diode array and laser-induced fluorescence detection.

The suitability of micellar electrokinetic chromatography (MEKC) coupled with diode array or laser-induced fluorescence (LIF) detection to analyze the four sulfur-containing excitatory amino acids (SEAA), homocysteine sulfinic acid (HCSA), homocysteic acid (HCA), cysteine sulfinic acid (CSA), and cysteic acid (CA) was investigated. 5-Carboxy-fluorescein succinimidyl ester was chosen as fluorescent reagent to derivatize HCSA, HCA, CSA, and CA. During method development, the yield of reaction dependent on pH and incubation time as well as the stability of the products were analyzed. The maximum yield was obtained after 30 min using a 0.1 M borate buffer (pH 8.9) as derivatization buffer. Each labeled amino acid exhibited high stability at room temperature over a period of 5 days. Baseline separation of labeled HCSA, HCA, CSA, and CA was obtained using a buffer consisting of 0.1 M borate, 50 mM sodium dodecyl sulfate (SDS), and 5% v/v methanol (pH 9.0). By applying LIF detection, limits of detection ranged from 0.9 x 10(-10) M for HCSA to 6.0 x 10(-10) M for CA, respectively. Slightly modified separation conditions enabled the analysis of SEAA in cerebrospinal fluid in the presence of the neurotransmitters glutamate and aspartate. In conclusion, MEKC coupled with LIF detection is a suitable technique for the simultaneous and sensitive analysis of SEAA. Further work will focus on the validation of the method with cerebrospinal fluid as sample matrix.

A gene mutated in nephronophthisis and retinitis pigmentosa encodes a novel protein, nephroretinin, conserved in evolution.

Nephronophthisis (NPHP) comprises a group of autosomal recessive cystic kidney diseases, which constitute the most frequent genetic cause for end-stage renal failure in children and young adults. The most prominent histologic feature of NPHP consists of development of renal fibrosis, which, in chronic renal failure of any origin, represents the pathogenic event correlated most strongly to loss of renal function. Four gene loci for NPHP have been mapped to chromosomes 2q13 (NPHP1), 9q22 (NPHP2), 3q22 (NPHP3), and 1p36 (NPHP4). At all four loci, linkage has also been demonstrated in families with the association of NPHP and retinitis pigmentosa, known as "Senior-Løken syndrome" (SLS). Identification of the gene for NPHP type 1 had revealed nephrocystin as a novel docking protein, providing new insights into mechanisms of cell-cell and cell-matrix signaling. We here report identification of the gene (NPHP4) causing NPHP type 4, by use of high-resolution haplotype analysis and by demonstration of nine likely loss-of-function mutations in six affected families. NPHP4 encodes a novel protein, nephroretinin, that is conserved in evolution--for example, in the nematode Caenorhabditis elegans. In addition, we demonstrate two loss-of-function mutations of NPHP4 in patients from two families with SLS. Thus, we have identified a novel gene with critical roles in renal tissue architecture and ophthalmic function.

Mapping of gene loci for nephronophthisis type 4 and Senior-Løken syndrome, to chromosome 1p36.

For nephronophthisis (NPHP), the primary genetic cause of chronic renal failure in young adults, three loci have been mapped. To identify a new locus for NPHP, we here report on total-genome linkage analysis in seven families with NPHP, in whom we had excluded linkage to all three known NPHP loci. LOD scores >1 were obtained at nine loci, which were then fine mapped at 1-cM intervals. Extensive total-genome haplotype analysis revealed homozygosity in one family, in the region of the PCLN1 gene. Subsequent mutational analysis in this gene revealed PCLN1 mutations, thereby allowing exclusion of this family as a phenocopy. Multipoint linkage analysis for the remaining six families with NPHP together yielded a maximum LOD score (Z(max)) of 8.9 (at D1S253). We thus identified a new locus, NPHP4, for nephronophthisis. Markers D1S2660 and D1S2642 are flanking NPHP4 at a 2.9-cM critical interval. In one family with NPHP4, extensive genealogical studies were conducted, revealing consanguinity during the 17th century. On the basis of haplotype sharing by descent, we obtained a multipoint Z(max) of 5.8 for D1S253 in this kindred alone. In addition, we were able to localize to the NPHP4 locus a new locus for Senior-Løken syndrome, an NPHP variant associated with retinitis pigmentosa.