Cephalosporins increase the renal clearance of methotrexate and 7-hydroxymethotrexate in rabbits

Summary Anaesthetized rabbits were infused with methotrexate (MTX; 30 μg × kg⁻¹ × min⁻¹ for 4h. Constant plasma concentrations of MTX and its main metabolite 7-hydroxymethotrexate (7-OH-MTX) were achieved 40–60 min after the start of the infusion. In all, 50% of the infused MTX was eliminated by the kidney; another 15%–30% was hydroxylated and excreted as 7-OH-MTX in the urine. A concomitant infusion of penicillin G (3.96 mg × kg⁻¹ × min⁻¹) decreased the renal clearance of MTX and 7-OH-MTX, probably by competitive antagonism at the common tubular secretion site. In contrast, the four cephalosporins ceftriaxone, ceftazidime, ceftizoxime and cefoperazone all increased the renal clearance of MTX and 7-OH-MTX. At similar plasma concentrations, ceftriaxone and ceftazidime were almost equipotent, ceftizoxime was less effective and cefoperazone seemed to have a biphasic effect, depressing the clearance of MTX and 7-OH-MTX at higher drug concentrations. The effects are best explained by an inhibition of the tubular reabsorption of the cytostatic and its metabolite. The results suggest that cephalosporins are a better choice than penicillin for antibiotic treatment during MTX therapy.     

Pharmacokinetics of methotrexate and 7-hydroxy-methotrexate in rabbits

Summary In rabbits the IV kinetics of MTX (1.33, 4 and 12 mg/kg) could be described by a linear three-compartment model with a terminal half-life between 2.4 and 3.6 h. During 8 h 50% of the dose was excreted into urine in unchanged form and 15% as the metabolite 7-OH-MTX. These fractions remained constant with increasing dose. In continuous infusion experiments (9–900 g/kg x min MTX IV) a decrease of the renal MTX clearance with increasing plasma concentration was observed. This effect was nearly compensated by an increase of the extrarenal MTX clearance. After short-term infusion of 7-OH-MTX (4 mg/kg) a biexponential decline of 7-OH-MTX plasma concentrations was observed with a terminal half-life of 0.45 h. About 80% of the dose was regained from urine during 5 h. From the combined pharmacokinetic data a linear model was constructed for the calculation of 7-OH-MTX plasma concentrations after short-term MTX infusion. For the first 4 h after MTX application the predicted values were in good accordance with the 7-OH-MTX concentrations actually measured.     

Influence of piperacillin on the pharmacokinetics of methotrexate and 7-hydroxymethotrexate

The influence of concomitant administration of piperacillin (PIP) on the pharmacokinetic parameters of methotrexate (MTX) and 7-hydroxymethotrexate (7-OH-MTX) was studied in rabbits. Six rabbits received an initial i.v. bolus (0.21 mg kg⁻¹) followed by a constant-rate i.v. infusion of the drug (5 μg min⁻¹ kg⁻¹) for 240 min. The PIP dose (30 mg kg⁻¹) was repeated every 30 min until the end of the infusion period. The control group consisted of four rabbits treated the same way except for the addition of PIP. There were significant increases in the mean residence times found for MTX (MRT_inf) and 7-OH-MTX (MRT_m,inf) following PIP administration. Concomitant administration of PIP with MTX also produced significant 1.5- and 2.8-fold increases in the area under the curve of MTX and 7-OH-MTX, respectively. The total body clearance of MTX and the operative total body clearance of 7-OH-MTX significantly decreased, but in a less than proportional manner. The study demonstrates that the interaction between MTX and PIP is mainly due to the reduced clearance of both MTX and 7-OH-MTX combined with a slight increase in the formation clearance of the metabolite. Received: 9 March 1998 / Accepted: 14 May 1998     

Theoretically required urinary flow during high-dose methotrexate infusion

Summary The renal excretion of methotrexate (MTX) and its major metabolite 7-hydroxymethotrexate (7-OH-MTX) was analysed in 12 children with malignancies during 52 courses of high-dose methotrexate (H-D-MTX) infusion at dosages ranging from 0.7 to 8.4 g/m².The peak concentrations of both MTX and 7-OH-MTX exceeded the aqueous solubilities of these compounds at low pH (6.0). The cumulative MTX excretion in urine was 75%–98% of the administered amount of MTX, and the cumulative 7-OH-MTX excretion in the urine was 3%–15%. The theoretically required urinary flow (TRUF) was estimated as the minimum urine volume needed for complete resolution of MTX and its metabolites in urine. TRUF during MTX infusion from 0 to 6 h and from 6 to 12 h was correlated with the dosage of MTX, and these values were 0.1–1.8 ml/min/m² at pH 7.0, 0.5–11.1 ml/min/m² at pH 6.0, and 1.9–42.2 ml/min/m² at pH 5.0 with dosages of 0.7 to 8.4 g/m². The value of the theoretically required urinary flow is important to ensure adequate hydration and the optimum alkalinization schedule for massive MTX infusion.     

Kinetics of 7-hydroxy-methotrexate after high-dose methotrexate therapy

Summary Kinetics of 7-hydroxy-methotrexate (7-OH-MTX) excretion after high-dose methotrexate (MTX) (12 g/m²) therapy were monitored in 93 consecutive drug cycles of 19 adolescent patients with osteosarcoma. A reversed-phase HPLC method was used. Serum elimination was found to be monophasic with a mean half-life of 5.5 h. Shortly after the 4-h MTX infusion period 7-OH-MTX levels exceeded those of the parent compound. By 12 h after MTX infusion 7-OH-MTX levels were 16.5 times higher than those of MTX itself.Autostimulation of MTX metabolism leading to enhanced 7-OH-MTX production after repeated drug cycles was not observed. The production of 7-OH-MTX decreased significantly from the first to the last high-dose MTX cycle of the adjuvant chemotherapy protocol.     

Renal and hepatic toxicity after high-dose 7-hydroxymethotrexate in the rat

To examine directly the hepatic and renal toxicity of 7-hydroxymethotrexate (7-OH-MTX) without interference of the parent compound methotrexate (MTX), we purified and gave 100 mg/kg 7-OH-MTX to rats, a dose resulting in serum levels of 7-OH-MTX comparable with those achieved in the clinic after the administration of high-dose MTX (HD-MTX). After only 5 h, the 7-OH-MTX-treated rats demonstrated 2.6-fold increases in serum creatinine values and 2-fold elevations in serum aspartate aminotransferase (ASAT) levels as compared with the controls. Morphologic evidence of toxicity, however, was apparent only in the kidneys. Intraluminal cellular debris containing membranous material and deteriorated organelles was seen, but no precipitate of the delivered drug. The peak serum concentration of 7-OH was up to 939 M, and concentrations of 7-OH-MTX declined triphasically, showing at1/2 value of 2.45 min, at1/2 value of 30.5 min, and a terminal half-life (t1/2) of 240 min. The total clearance value was 14.5 ml min^–1 kg, and the postdistributional volume of distribution (V) was 5070 ml/kg. Our results may indicate a direct toxic effect of 7-OH-MTX on kidney and liver cells.This study was supported financially by the Norwegian Cancer Society     

Protection against cisplatin nephrotoxicity by prochlorperazine

Summary In pentobarbital anesthetized rats that received 4 mg/kg i.v. methotrexate (MTX) or 7-hydroxymethotrexate (7-OH-MTX), the pharmacokinetics of the two drugs were similar. Plasma concentrations of both drugs declined biexponentially, with terminal half-lives of 90.6 min for MTX and 97.2 min for 7-OH-MTX. The total clearance values were 9.2 and 9.6 mlxkg^-1xmin^-1, respectively. With MTX, 48.2% of the dose was excreted in the urine within 200 min and another 31.6% was recovered from the bile; 5.8% was metabolized to 7-OH-MTX and appeared in the bile. Plasma concentrations of the metabolite 7-OH-MTX after MTX administration were below the detection limit. Injected 7-OH-MTX was predominantly excreted into the bile (72.8% of the dose); only 11.2% could be recovered from the urine. Differences between the physicochemical properties of MTX and 7-OH-MTX or different affinities for active transport systems may account for the unequal importance of these two excretion pathways for the two compounds.     

Inhibition of 7-hydroxymethotrexate formation by amsacrine

Summary The inhibition of methotrexate (MTX) biotransformation to 7-hydroxymethotrexate (7-OH-MTX) by 4-(9-acridinylamino)-methanesulfon-m-anisidide (mAMSA) was studied in bile-drained rats in vivo and in incubates of isolated rat hepatocytes and rat-liver homogenate in vitro. In vivo, i.v. administration of 10 mg/kg mAMSA prior to [³H]-MTX infusion (50 mg/kg) led to a significant alteration in 7-OH-MTX kinetics. 7-OH-MTX peak concentrations and AUC in bile and serum were reduced by 75% and the recovery of MTX as 7-OH-MTX in bile and urine decreased by 70%, whereas MTX pharmacokinetics remained unaltered. In suspensions of isolated hepatocytes. 10 m mAMSA led to a 54% decrease in 7-OH-MTX formation. However, the hepatocellular influx and efflux of MTX was not perturbed by mAMSA. Preincubation of rat-liver homogenates with 1.25–10 m mAMSA reduced the formation of 7-OH-MTX by up to 73%. mAMSA appeared to inhibit MTX hydroxylation competitively, exhibiting aK _iof 3 m. Due to its inhibition of the MTX-oxidizing system, mAMSA may be beneficial in combination chemotherapy with MTX by reducing 7-OH-MTX-associated toxicity and, possibly, enhancing the cytotoxic effects of MTX.This study was financially supported by the Norwegian Cancer Society and the Erna and Olav Aakre Foundation for Cancer Research     

Dose-dependent pharmacokinetics of methotrexate and 7-hydroxymethotrexate in the rat in vivo

The pharmacokinetics of methotrexate (MTX) and 7-hydroxymethotrexate (7-OH-MTX) in bile, urine, and serum was studied in rats in vivo after short-time infusions of 10, 50, 250, and 1000 mg/kg MTX. All animals were anesthetized and drained of bile during experiments. The biliary secretion rate of MTX approached saturation when serum MTX levels surpassed 700-800 microM, causing a significant reduction in biliary recovery as the parent compound (49 to 32%) at MTX doses exceeding 50 mg/kg. The hepatic metabolism of MTX to the 7-hydroxy metabolite was not saturated at the doses used. Serum MTX pharmacokinetics demonstrated dose dependency, inasmuch as doses exceeding 10 mg/kg were accompanied by a reduced total body clearance (Clr) and biliary clearance (ClB). A significant finding in relation to acute hepatotoxicity reported after high-dose MTX in humans was the occurrence of cholestasis 30-90 min after drug infusion and the observation of macroscopic precipitations in the bile duct in five of six animals treated with 1000 mg/kg MTX. In these five animals, cessation of bile secretion occurred at similar bile 7-OH-MTX levels [9800 +/- 1100 (SD) microM], while the single rat that secreted bile throughout the experiment had a 5-fold lower peak 7-OH-MTX concentration in bile. Analysis of biliary precipitates formed in vivo and in vitro found 7-OH-MTX to constitute 97% and MTX 3% of the drug content of the precipitated material.     

Methotrexate administered by 6-h and 24-h infusion: a pharmacokinetic comparison

Summary The pharmacokinetics of 8 g/m² methotrexate (MTX) was compared following short (6 h) and long (24 h) infusions of the drug to 11 children with osteogenic sarcoma (OS; 42 infusion) and 28 children with acute lymphoblastic leukemia (ALL: 118 infusions), respectively. No difference was observed in the first-phase half-life, in systemic clearance or in the volume of distribution of the drug (P>0.05). The concentration of MTX at the end of the infusion was 4-fold higher when the drug was given over only 6 h. However, patients receiving 24-h infusions had 9-fold higher levels by 24 h after the beginning of the infusion. The area under the data curve from start of the MTX infusion until the beginning of folinic acid rescue administration was significantly higher in patients with osteogenic sarcoma (6-h infusions), while the area under the log-data curve was significantly longer in the ALL group (24-h infusions) for the same period. The latter parameter is considered to be characteristic for the concentration-time-effect relationship. The longer duration of MTX administration (with delayed rescue) is thought to be more beneficial from the pharmacokinetic aspect. Patients with osteogenic sarcoma had significantly lower concentrations of MTX at the end of their last treatment with MTX than at the end of the first infusion. Patients developing MTX toxicity had shorter half-lives of MTX in the beta phase. It is suggested that cisplatin induced tubular loss of MTX and folinic acid is responsible for these observations. A wider application of clinical pharmacologic findings in the practice of the administration of cytostatics is indicated.The work reported in this paper was supported by a research grant from the Norwegian Cancer Society (Landsforeningen mot Kreft)     

High-dose 7-hydroxymethotrexate: Acute toxicity and lethality in a rat model

To elucidate mechanisms for methotrexate (MTX)-induced renal and hepatic toxicity, we investigated the acute effects of bolus plus continuous infusion of up to 0.4 g/kg 7-hydroxymethotrexate (7-OH-MTX) in the rat. We demonstrate for the first time in any species the occurrence of acute lethal toxicity within a few hours after 7-OH-MTX administration. Serum concentrations of 7-OH-MTX measured at the time of death were 1.4 mM (mean), about one-half of those achieved in some patients after infusion of high-dose MTX (HD-MTX) in the clinic. The data suggest an approximate LD50 (the dose lethal to 50% of the study population) of 0.3 g/kg and a steep dose/ lethality curve for 7-OH-MTX. Moreover, acute renal and hepatic toxicity occurred as evidenced by severe morphological findings and increased serum levels of creatinine and liver transaminases. In all rats subjected to continuous infusion of 7-OH-MTX, yellow microscopic precipitations were apparent in the kidney tubules. Crystallization was also seen in bile ducts of the liver in some of the rats. These results further support that the formation of 7-OH-MTX is disadvantageous and that reported attempts to prevent its formation during MTX treatment are warranted.     

Maximum tolerated doses of methotrexate and 7-hydroxy-methotrexate in a model of acute toxicity in rats

Purpose: After more than 50 years of methotrexate (MTX) treatment of acute lymphoblastic leukaemia (ALL), it is currently believed that as long as dose escalations are followed by adequate leucovorin rescue guided by monitoring MTX serum concentrations, hydration and urinary alkalinization, high-dose MTX (HD-MTX) can be tolerated without life-threatening toxicity. However, our recent experimental animal studies of the major metabolite of MTX, 7-OH-MTX, indicate that this concept may have some limitations. Animals with levels of 7-OH-MTX of 1 mM, which is below the levels routinely found in patients on HD-MTX, demonstrate intolerable toxicity and some animals die within 8 h. Electron microscopy indicates that endothelial cell and platelet functions are perturbed. Since animal data are lacking, and interspecies differences not known, we wanted to investigate the maximum tolerated doses of MTX and 7-OH-MTX in a rat model of short-term effects. The maximum tolerated dose was chosen instead of LD₅₀ for reasons of animal welfare. Methods: We infused MTX and 7-OH-MTX into anaesthetized male Wistar rats and monitored the animals for 8 h. The drugs were given as a bolus plus continuous infusion. The dose-finding ranges were 1.8–11.3 g/kg MTX and 0.1–1.2 g/kg 7-OH-MTX. Results: The maximum tolerated dose was between 3 and 5 g/kg for MTX and lower than 0.1 g/kg for 7-OH-MTX. The mean serum concentrations of MTX and 7-OH-MTX in animals that did not survive the 8-h period were 21.9 and 1.6 mM, respectively. The animals that received the highest MTX or 7-OH-MTX doses and concentrations died after sudden reductions in heart rate and blood pressure. Conclusions: We demonstrated a lower maximum tolerated dose of 7-OH-MTX than of MTX in rats after 8 h. The 7-OH-MTX concentrations were in the therapeutic range after HD-MTX. If the rat/human interspecies differences are not large, our data may indicate that HD-MTX regimens should not be further dose intensified, due not so much to the effects of MTX as to those of 7-OH-MTX. Received: 28 July 1999 / Accepted: 25 January 2000     

7-Hydroxymethotrexate concentrations in serum and cerebrospinal fluid of children with acute lymphoblastic leukemia

Summary Concentrations of methotrexate (MTX) and 7-hydroxymethotrexate (7-OH-MTX) were determinded by HPLC in the serum and cerebrospinal fluid (CSF) of 29 children with acute lymphoblastic leukemia. CSF and serum samples were obtained at the end of 104 infusions of MTX given in a dose range of 0.5–8.0 g/m². Concentrations, distribution ratios in serum and CSF for MTX and 7-OH-MTX, and the metabolic index were analyzed with regard to the MTX dose, age and clinical state of the patients. A wide inter-patient (2- to 12-fold) but narrower (1,1- to 3,5-fold) intra-patient variability of the concentrations was observed. A dose-proportional increase in the metabolite concentration was found in serum. On the other hand, the elevation of the level of metabolite in CSF was less than porportional to the dose. The CSF/serum distribution data suggest the existence of a saturable carrier system for MTX and 7-OH-MTX between serum and CSF that has lower affinity for 7-OH-MTX. No correlation was found between concentrations of MTX and 7-OH-MTX in the serum of patients receiving the same dose of MTX. No significant difference was observed in the values for metabolic index between relapsed patients and those who were in continuous complete remission. A significant correlation was found between age and metabolic index: the younger the patient, the higher the metabolite concentration measured in serum.     

Pharmacokinetics of methotrexate and 7-hydroxy-methotrexate in plasma and bone marrow of children receiving low-dose oral methotrexate

Summary The absorption, distribution, and elimination kinetics of low-dose p.o. methotrexate (MTX) were repeatedly studied in 19 children during maintenance treatment of childhood acute lymphoblastic leukemia. Plasma concentrations, urinary elimination, and bone marrow concentrations of MTX and 7-hydroxymethotrexate (7-OH-MTX) were monitored during 24 h following a routime p.o. dose (30 mg/m²) using high-pressure liquid chromatography. Significant interindividual variability was found in time to peak concentration (30–180 min), peak concentration (0.41–2.77 M), and to a lesser extent the half-lives (t_1/2: 32.8–86.1 min; t_1/2: 43.6–350.0 min; t_1/2 absorption: 25.2–60.3 min) and plasma area under the concentration-time curve from zero to infinity (195.6–818.5 M.min). Significant amounts of 7-OH-MTX were detected in plasma, with a mean area under the concentration-time curve from zero to infinity of 208 M.min compared with 365.6 M.min for MTX. High concentrations of 7-OH-MTX were present in bone marrow 24 h after oral MTX (15/19 patients) and were at least five fold those in plasma and three fold the concentration of MTX in bone marrow. In four patients occasionally neither MTX nor metabolite could be detected. Repeated examination of these pharmacokinetic parameters in plasma and bone marrow showed that the intraindividual variability was small.This study was supported by the Netherlands Cancer Foundation Koningin Wilhelmina Fonds     

Late-onset delayed excretion of methotrexate

Pleural effusions, ascites, and renal dysfunction decrease the plasma excretion of methotrexate (MTX). However, it is not known what effect these complications have on MTX clearance when they arise after the plasma MTX concentration has fallen to an undetectable level. We describe the clinical course and pharmacokinetics of MTX in a patient with acute lymphoblastic leukemia who experienced pleural effusions, ascites, and renal failure during the weeks after treatment with high-dose MTX (1.63 g/m² i.v. over 24 h). The patients normal initial MTX clearance rate (107 ml/min/m²) was consistent with his undetectable plasma level of MTX on day 9 after the infusion. His plasma MTX concentration then gradually increased as his renal function declined, reaching a peak of 0.72 M on day 15. This unusual finding of an undetectable plasma MTX concentration that subsequently rose to persistent, potentially toxic levels was explained only by a pharmacokinetic model that accounted both for a third space at the time of treatment and for the subsequent decrease in the systemic elimination rate. Therefore, the finding of a physiologic third space during MTX administration combined with the detection of renal dysfunction in the following weeks should be an indication for prolonged therapeutic drug monitoring.     

Interactions between 7-hydroxymethotrexate and methotrexate at the cellular level in the Ehrlich ascites tumor in vitro

Studies were undertaken to characterize the cellular pharmacology of 7-hydroxymethotrexate (7-OH-MTX) in Ehrlich ascites tumor cells, compare it to that of methotrexate (MTX), and define the interactions between the parent compound and its catabolite. Transport of 7-OH-MTX is mediated by the MTX-tetrahydrofolate cofactor carrier, with a Km of 9 microM in comparison to the MTX Km of 5 microM. Both compounds mutually inhibit their influx and steady-state levels of free drug accumulated. While influx of 7-OH-MTX is slower than influx of MTX, 7-OH-MTX efflux is likewise slower, so that the steady-state level of 7-OH-MTX achieved is comparable to that of MTX. Influx of 7-OH-MTX is inhibited by extracellular 5-formyltetrahydrofolate and trans-stimulated in cells preloaded with this tetrahydrofolate cofactor. The energetics of 7-OH-MTX transport is similar to that of MTX in the influx and net transport are stimulated by sodium azide, while net transport is reduced by glucose. As observed for MTX, 7-OH-MTX transport is sensitive to the anionic composition of the extracellular compartment and was shown to be inhibited by organic and inorganic phosphates. 7-OH-MTX does not, alone, inhibit [3H]deoxyuridine incorporation into DNA at concentrations of up to 50 microM. However, the catabolite reduces MTX inhibition of deoxyuridine metabolism, presumably due to the reduction in the free level of intracellular MTX achieved. These findings support the possibility that when 7-OH-MTX accumulates to high levels relative to MTX in clinical regimens, it may modulate the pharmacological effects of MTX.     

Formation of 7-hydroxymethotrexate polyglutamyl derivatives and their cytotoxicity in human chronic myelogenous leukemia cells, in vitro

The rapid synthesis of poly-gamma-glutamyl derivatives of 7-hydroxymethotrexate (7-OH-MTX) and their selective intracellular retention are reported in human chronic myelogenous leukemia cells, K-562. After a 30-min exposure to 5 microM [3H]7-OH-MTX, three different polyglutamyl derivatives were detected by high-performance liquid chromatography. When extracellular 7-OH-MTX was removed, the 7-OH-MTX diglutamate level declined slowly in comparison to the monoglutamate, but the higher polyglutamyl derivative levels increased. Within 10 min after exposure of cells to 7-OH-MTX, the level of these polyglutamyl derivatives far exceeds the dihydrofolate reductase binding capacity. Gel filtration or charcoal binding analysis followed by high-performance liquid chromatography analysis of the bound component showed intracellular binding of virtually all 7-OH-MTX tetraglutamate at a level 4-fold higher than that of the dihydrofolate reductase binding capacity. No bound 7-OH-MTX diglutamate or triglutamate could be detected. Treatment of the 7-OH-MTX tetraglutamate: protein complex with 100 microM unlabeled methotrexate (MTX) for 15 min resulted in only a partial dissociation of this complex to an extent compatible with the dihydrofolate reductase level. The residual 7-OH-MTX tetraglutamate remained bound to a site with a molecular weight of approximately 25,000 to 35,000 as assessed by Bio-Gel P-60 analysis and could not be displaced by folic acid, 5-formyltetrahydrofolate, 7-OH-MTX, or the tetraglutamate of MTX. 7-OH-MTX and MTX cytotoxicities were compared by clonogenic assay in agar and by their effects on cell growth. After a 2-hr exposure, the 50% inhibitory concentrations for 7-OH-MTX and MTX in cells growing in agar were 10(-5) and 10(-6) M, respectively. A 10-fold difference in cytotoxicity was also observed in cells growing in suspension. Continuous exposure to glycine: adenosine: thymidine completely protects cells from a sustained exposure to 7-OH-MTX over the entire period of clonal growth. However, even a brief exposure to 7-OH-MTX also requires continuous exposure to glycine: adenosine: thymidine for protection. This suggests that, as observed for MTX, the 7-OH-MTX polyglutamyl derivatives that are retained within the cells have a sustained cytotoxic effect after the monoglutamate is removed.     

Formation and elimination of 7-hydroxymethotrexate in the rat in vivo after methotrexate administration

Bile, urine, and serum concentrations of methotrexate (MTX) and 7-hydroxy-methotrexate (7-OH-MTX) were monitored in rats in vivo following a short-time infusion of 10 mg/kg [3H]MTX. The experiments were performed in one group of anesthetized, bile-drained rats and in two control groups, one anesthetized and one unanesthetized, that were not bile-drained. Peak biliary levels of MTX (3.8 x 10(-3) M) and 7-OH-MTX (1.8 x 10(-4) M) appeared within 15 min after cessation of infusions. For two log ranges of serum MTX concentrations, biliary levels remained 180-fold higher. High bile 7-OH-MTX levels appeared few min after start of MTX administration, and were 720 times higher than the peak serum concentrations, indicating that the liver is a major site of 7-OH-MTX formation in the rat. 7-OH-MTX concentrations in bile declined monophasically with a half-life of 29.4 min, while MTX showed a biphasic elimination with initial and second phase half-lives of 23.1 and 86.4 min, respectively. Bile was the major excretory route for MTX and 7-OH-MTX, with 50% of the dose recovered as the parent compound and 3.6% as the metabolite. There was no difference in urinary recovery of MTX in bile-drained and control animals, indicative of insignificant enterohepatic circulation of MTX. This was further corroborated by the finding of just 2.1% urinary recovery of MTX in rats who received previously collected MTX-containing bile through a duodenal catheter. Serum concentration curves were analyzed according to a three-compartment open model with an initial elimination half-life of 1.7-3.3 min, a second phase half-life of 15.4-21.0 min, and a terminal phase half-life of 119-240 min. Our finding of 7-OH-MTX formation and high biliary levels of the metabolite in the rat, can be used as basis for studies of interactions with in vivo MTX conversion to the 7-hydroxy metabolite.     

Renal clearance of methotrexate in man during high-dose oral and intravenous infusion therapy

Summary The renal excretion and clearance of methotrexate (MTX) following high-dose (800 mg) therapy followed by folinic acid rescue was studied in 12 patients (2 female, 10 male): the mean age was 49.3±5.5 (SE), weight 68.6±3.9 (SE) and body surface area 1.8±0.1 m². Plasma and urine were collected over 154 h at intervals of 2–24 h, and the collection times, volume, and pH of urine samples recorded. Total MTX concentrations in urine and plasma were measured by the highly specific competitive protein-binding assay method. Plasma and urinary creatinine levels were measured on an SMA-12 autoanalyser. The renal clearance of MTX was calculated for each urine collection period. Following oral administration, clearance values during the first 6 h were high at 257±8.3 (ml/min), followed by a trough in clearance of 27.9±4.2 (ml/min) in the 20- to 30-h period. This was followed by a secondary rise of MTX renal clearance to 180.4±14.6 ml/min during the 68- to 84-h period and again to 84.9±17.1 ml/min between 84 and 112 h. In the last two periods it rose to 209±57.9 ml/min. Similar fluctuations were seen following IV administration. The changes in clearance were statistically significant at the P<0.005 level. It is suggested that high concentrations of MTX in the renal tubules result in inhibition of carrier protein synthesis, leading to a fall in active tubular secretion. When MTX concentrations fall the tubular cell recovers and a secondary rise in renal clearance occurs, leading to cyclical changes in MTX elimination.     

Nonlinear renal clearance of ultrafilterable platinum in patients treated with cis-dichlorodiammineplatinum (II)

Summary Nonlinear renal clearance of ultrafilterable platinum was observed in 5 of 7 patients given cis-dichlorodiammineplatinum (II) in doses of 50–140 mg/m² by short-term infusion (2h). Average renal clearance determined during and 24 h after infusion ranged from 100 to 543 ml/min and always exceeded creatinine clearance, suggesting that ultrafilterable platinum was renally secreted. Saturable tubular reabsorption was postulated on the basis that renal clearance was highest at peak plasma and urinary levels and fell as the levels declined. Although an overall relationship between dose and renal clearance was not apparent, one patient receiving the highest dose (140 mg/m²) had elevated average renal clearance (485 ml/min), probably associated with saturation of reabsorption, whereas a patient receiving 50 mg/m² had the lowest average renal clearance (100 ml/min), indicating that either active secretion was lower, or tubular reabsorption was saturated. One patient also showed urine-flow-dependent changes in renal clearance. Four patients had transient rises in ultrafilterable platinum levels, which were attributed to changes in renal tubular reabsorption. The results suggest that renal clearance of ultrafilterable platinum is probably dependent on cis-DDP dose, urine flow rate, and individual variability in the extent of active secretion and tubular reabsorption. A sensitive HPLC method was applied and ultrafilterable platinum was detected in the plasma of all patients 24 h after infusion. Renal tubular reabsorption may result in prolonged plasma levels of ultrafilterable platinum, which could contribute to the drug's antitumour effect.