Effect of probenecid on ventricular cerebrospinal fluid methotrexate pharmacokinetics after intralumbar administration in nonhuman primates.

PURPOSE: Intrathecal methotrexate (MTX) achieves high concentrations in the cerebrospinal fluid (CSF) following intralumbar administration. However, peak ventricular CSF MTX concentrations are highly variable and are < 10% of those achieved with intraventricular dosing. The objectives of this study were to evaluate the effect of intralumbar and intravenous probenecid on ventricular CSF MTX concentrations after intralumbar administration of MTX, and to compare the pharmacokinetics of MTX after intralumbar and intraventricular administration. METHODS: Nonhuman primates (Macaca mulatta) with permanently implanted catheters in the lateral and fourth ventricles received 0.5 mg intraventricular (lateral ventricle) MTX, or 0.5 mg intralumbar MTX with and without intralumbar or intravenous probenecid. Animals were kept prone for 1 h after MTX administration, and ventricular CSF was sampled up to 48 h from a fourth ventricular Ommaya reservoir. MTX concentrations were measured using the dihydrofolate reductase enzyme inhibition assay. Area under the ventricular CSF MTX concentration-time curve (AUC) was used as a measure of MTX exposure. RESULTS: Peak ventricular CSF MTX concentrations and AUCs were highly variable after intralumbar MTX administration. Ventricular CSF MTX AUCs increased by a mean of 3.2-fold after the addition of intralumbar probenecid. Intravenous administration of probenecid did not result in an increase in ventricular CSF MTX AUCs. Asymptomatic pleocytosis was observed in all animals after intralumbar probenecid administration. Ventricular CSF MTX concentrations and AUCs were less variable after intraventricular administration of MTX. CONCLUSION: The administration of intralumbar but not intravenous probenecid increases the ventricular CSF MTX exposure after intralumbar MTX administration.

Plasma pharmacokinetics and cerebrospinal fluid penetration of hypericin in nonhuman primates.

UNLABELLED: Hypericin, a polycyclic aromatic dianthroquinone, is a natural pigment derived from the plant Hypericum perforatum (St John's Wort). The compound has been synthesized and shown to inhibit the growth of malignant glioma cell lines in vitro via inhibition of protein kinase C. Oral hypericin has entered clinical trials in adults with recurrent malignant glioma. PURPOSE: The present study was performed to characterize the plasma pharmacokinetics (PK) and cerebrospinal fluid (CSF) penetration of hypericin in nonhuman primates. METHODS: Hypericin was administered as an intravenous bolus dose of 2 mg/kg (n = 3) or 5 mg/kg (n = 1). Plasma and CSF (ventricular or lumbar) were sampled prior to administration and at frequent intervals for up to 50 h after administration of the drug. Hypericin concentrations in plasma and CSF were determined using a specific reverse-phase HPLC assay. RESULTS: Mean peak plasma concentration of hypericin following the 2 mg/kg dose was 142 +/- 45 microM. Elimination of hypericin from plasma was biexponential, with an average alpha half-life of 2.8 +/- 0.3 h and average terminal half-life of 26 +/- 14 h. CONCLUSIONS: The 2 mg/kg dose in the nonhuman primate was sufficient to maintain plasma concentrations above 10 microM (the in vitro concentration required for growth inhibition of human glioma cell lines) for up to 12 h. No hypericin was detected in the CSF of any animal (lower limit of detection 0.1 microM); the CSF penetration is therefore less than 1%. A severe dose-limiting photosensitivity skin rash was seen at the 5 mg/kg dose level.

Plasma and cerebrospinal fluid pharmacokinetics of O6-benzylguanine and analogues in nonhuman primates.

O6-Benzylguanine (BG) is a potent, specific inactivator of the DNA repair protein, O6-alkylguanine-DNA alkyltransferase, that enhances the sensitivity of tumor cell lines and tumor xenografts to chloroethylnitrosoureas. To search for BG analogues with greater penetration into the cerebrospinal fluid (CSF), we evaluated plasma and CSF pharmacokinetics of BG, 8-aza-O6-benzylguanine (8-azaBG), O6-benzyl-8-bromoguanine (8-BrBG), O6-benzyl-8-oxoguanine (8-oxoBG), O6-benzyl-8-trifluoromethylguanine (8-tfmBG), and O6-benzyl-2'-deoxyguanosine (B2dG) after i.v. administration of 200 mg/m2 of drug through an indwelling Ommaya reservoir in a nonhuman primate model. BG and its analogues were quantified in plasma and CSF using reverse-phase high-performance liquid chromatography assays. The plasma clearances of the four 8-substituted BG analogues were similar (0.04-0.06 l/h/kg), but half-lives ranged from <2 to >24 h. BG was converted to 8-oxoBG, an equally potent O6-alkylguanine-DNA alkyltransferase inactivator, and the elimination of 8-oxoBG was much slower than that of BG. As a result, the plasma area under the curve of 8-oxoBG was 3.5-fold greater than that of BG. B2dG was metabolized to BG and 8-oxoBG, but this pathway accounted for only 20% of B2dG elimination. The CSF penetration percentages (based on the ratio of AUC(CSF): AUCplasma) for BG, 8-azaBG, 8-oxoBG, 8-tfmBG, 8-BrBG, and B2dG were 3.2, 0.18, 4.1, 1.4, <0.3, and 2.0%, respectively. The CSF penetration of BG and its active metabolite 8-oxoBG is greater than the penetration of 8-azaBG, 8-BrBG, 8-tfmBG, and B2dG.

Pharmacokinetics and metabolism of the methotrexate metabolite 2, 4-diamino-N(10)-methylpteroic acid.

The novel methotrexate (MTX) rescue agent carboxypeptidase-G(2) (CPDG(2)) converts >98% of plasma MTX to 2, 4-diamino-N(10)-methylpteroic acid (DAMPA) and glutamate in patients with MTX-induced renal failure and delayed MTX excretion. DAMPA is eliminated more rapidly than MTX in these patients, suggesting nonrenal elimination. The pharmacokinetics and metabolism of DAMPA were studied in four nonhuman primates with reverse-phase HPLC with UV, photodiode array detection, and mass spectroscopy. The mean peak plasma DAMPA concentration was 51 microM and the plasma disposition was described by a three-compartment open model with first order elimination. The mean clearance of DAMPA was 1.9 l/kg/h and the mean terminal half-life was 51 min. Forty-six percent of the dose was excreted in the urine as parent compound. Three DAMPA metabolites, hydroxy-DAMPA, DAMPA-glucuronide, and hydroxy-DAMPA-glucuronide, were identified in plasma and urine. These metabolites also were identified in plasma from patients who received CPDG(2) as an MTX rescue agent. The cytotoxicity of DAMPA and its effect on MTX cytotoxicity were assessed in the Molt-4 human leukemic cell line. DAMPA was not cytotoxic and did not significantly alter the cytotoxicity of MTX. In nonhuman primates metabolism of DAMPA is a major route of DAMPA elimination, and metabolism underlies the more rapid elimination of DAMPA versus MTX in patients with MTX-induced renal dysfunction after administration of CPDG(2).

Effect of fluconazole on the pharmacokinetics of doxorubicin in nonhuman primates.

Antifungal prophylaxis in cancer patients who are undergoing chemotherapy is associated with prolonged neutropenia. We measured the effect of fluconazole on doxorubicin pharmacokinetics in nonhuman primates to determine if neutropenia is related to a pharmacokinetic interaction that delays the clearance of the chemotherapeutic agent. Fluconazole pretreatment had no effect on doxorubicin pharmacokinetics.

Effect of P-glycoprotein modulation with cyclosporin A on cerebrospinal fluid penetration of doxorubicin in non-human primates.

PURPOSE: P-glycoprotein (Pgp) is a transmembrane drug efflux pump that is expressed in multidrug-resistant cancer cells and in a variety of normal tissues, including brain capillary endothelial cells which comprise the blood-brain barrier. We studied the effects of the Pgp inhibitor, cyclosporin A (CsA), on the cerebrospinal fluid (CSF) penetration of the Pgp substrate, doxorubicin, in non-human primates. METHODS: The animals received doxorubicin alone (2.0 mg/kg i.v. over 60 min) or doxorubicin (1 mg/kg i.v. over 60 min) and CsA (loading dose 4.0 mg/kg i.v. over 2 h, followed by continuous infusion of 12 mg/kg per day over 48 h). Plasma and CSF were collected over 48 h and the doxorubicin concentration was measured by reverse-phase high-pressure liquid chromatography (HPLC) with fluorescence detection (detection limit 5 nM). A two-compartment model was fitted to the plasma concentration-time data. RESULTS: Pgp was demonstrated to be present in the epithelium of the choroid plexus by immunohistochemical methods, indicating that CSF drug penetration could be used as a surrogate for blood-brain barrier penetration. Steady state whole blood CsA concentrations, which were measured with a fluorescence-polarization immunoassay (TDX) that detects both CsA and its metabolites, ranged from 551-1315 microg/l at 24 h. The clearance of doxorubicin in four animals was reduced by 34%, 38%, 45% and 49% when given with CsA. The doxorubicin concentration in the CSF was <5 nM in all animals, both after doxorubicin alone and doxorubicin with CsA. CONCLUSIONS: The Pgp inhibitor, CsA, at a concentration that alters systemic clearance of doxorubicin, does not appear to significantly increase the CSF penetration of doxorubicin.

Methotrexate distribution within the subarachnoid space after intraventricular and intravenous administration.

PURPOSE: Intrathecal methotrexate achieves high concentrations in cerebrospinal fluid (CSF), but drug distribution throughout the subarachnoid space after an intralumbar dose is limited. The objective of this study was to quantify methotrexate distribution in CSF after intraventricular and intravenous administration and to identify factors that influence CSF distribution. METHODS: Nonhuman primates (Macaca mulatta) with permanently implanted catheters in the lateral and fourth ventricles received methotrexate by bolus injection (0.5 mg) and infusion (0.05 to 0.5 mg/day over 24 to 168 h) into the lateral ventricle, as well as intravenous infusions. CSF was sampled from the lumbar space, fourth ventricle and the subarachnoid space at the vertex. Methotrexate in CSF and plasma was measured with the dihydrofolate reductase inhibition assay. RESULTS: After bolus intraventricular injection, methotrexate exposure in lumbar CSF ranged from 11% to 69% of that achieved in the fourth ventricle. During continuous intraventricular infusions, methotrexate steady-state concentrations (C(ss)) in lumbar CSF and CSF from the vertex were only 20% to 25% of the ventricular CSF C(ss). The dose, duration of infusion, and infusate volume did not influence drug distribution to the lumbar CSF, but probenicid increased the lumbar to ventricular C(ss) ratio, suggesting the involvement of a probenicid-sensitive transport pump in the efflux of MTX from the CSF. During the intravenous infusions, the ventricular methotrexate C(ss) was lower than the lumbar C(ss) and the C(ss) in CSF from the vertex. CONCLUSION: Methotrexate CSF distribution after intraventricular injection was uneven, and at steady-state CSF methotrexate concentrations were lower at sites that were more distant from the injection site.

The plasma pharmacokinetics and cerebrospinal fluid penetration of the thymidylate synthase inhibitor raltitrexed (Tomudex) in a nonhuman primate model.

PURPOSE: Raltitrexed (Tomudex), ZD1694) is a novel quinazoline folate analog that selectively inhibits thymidylate synthase. Intracellularly, raltitrexed is polyglutamated to its active form which can be retained in cells for prolonged periods. The pharmacokinetics of raltitrexed in plasma and cerebrospinal fluid (CSF) were studied in a nonhuman primate model. METHODS: Animals received 3 mg/m(2) (n = 1), 6 mg/m(2) (n = 3), or 10 mg/m(2) (n = 3) i.v. over 15 min, and frequent plasma samples were obtained over 48 h. CSF samples were drawn from an indwelling 4th ventricular Ommaya reservoir over 48 h. Plasma and CSF raltitrexed concentrations were measured with a novel, sensitive enzyme inhibition assay with a lower limit of quantification of 0.005 microM. A three-compartment pharmacokinetic model was fitted to the raltitrexed plasma concentration-time data. RESULTS: The plasma concentration-time profile of raltitrexed was triexponential with a rapid initial decline and a prolonged terminal elimination phase (t(1/2) > 24 h), which was related to retention of raltitrexed in a deep tissue compartment. At the peak approximately 30% of the administered dose was in the deep tissue compartment, and 24 h after the dosing >20% of the administered dose remained in the body with >99% in the deep tissue compartment. The mean peak (end of infusion) plasma concentrations after the 3, 6, and 10 mg/m(2) doses were 1.5, 2.4 and 4.8 microM, respectively. The clearance of raltitrexed ranged from 110 to 165 ml/min per m(2), and the steady-state volume of distribution exceeded 200 l/m(2). The CSF penetration of raltitrexed was limited (0.6 to 2.0%) and drug could only be detected in the CSF following a 10 mg/m(2 )dose. CONCLUSIONS: The elimination of raltitrexed is triexponential with a prolonged terminal elimination phase. The pharmacokinetic profile is consistent with extensive polyglutamation and intracellular retention of ralitrexed. The three-compartment model presented here may be useful for the analysis of the pharmacokinetics of raltitrexed in humans.

Pharmacokinetics of O6-benzylguanine and its active metabolite 8-oxo-O6-benzylguanine in plasma and cerebrospinal fluid after intrathecal administration of O6-benzylguanine in the nonhuman primate.

O6-Benzylguanine (O6BG) irreversibly inactivates the single-turnover DNA repair protein alkylguanine-alkyltransferase. Thus, O6BG increases tumor-cell sensitivity to alkylating agents such as carmustine, lomustine, procarbazine, and temozolomide. We investigated the pharmacokinetic behavior of O6BG and O6-benzyl-8-oxoguanine (8-oxo-O6BG) in cerebrospinal fluid (CSF) and plasma after intraventricular administration of O6BG in a nonhuman primate model. In our study, three animals received a single 1-mg dose of O6BG into the lateral ventricle. CSF from the 4th ventricle and plasma samples were collected after administration, and O6BG and 8-oxo-O6BG concentrations were measured by high-performance liquid chromatography. Four additional animals received 1 mg of O6BG via the intralumbar route weekly for 6 weeks to assess the feasibility and toxicity of this route of administration. The peak O6BG CSF concentration was 412+/-86 microM, the t1/2 was 0.52+/-0.02 h, the clearance was 0.22+/-0.01 ml/min, and the area under the concentration-time curve was 319+/-15 microM x h in 4th ventricular CSF. The peak CSF concentration of 8-oxo-O6BG in CSF was 1.9+/-0.4 microM, the t1/2 was 0.76+/-0.03 h, and the area under the concentration-time curve was 5.0+/-1.1 microM x h. Both O6BG and 8-oxo-O6BG were detected in the plasma 0.5-3 h after intraventricular O6BG administration. The plasma peak concentration of O6BG was 0.4 microM at 30 min, and the concentration was <0.1 microM by 3 h. The plasma concentration of 8-oxo-O6BG was 0.2 microM at 30 min and 0.6 microM at 3 h. The animals tolerated the single intraventricular dose and 6 weekly intralumbar doses of O6BG without toxicity. We concluded that intrathecal administration of O6BG is well tolerated in the nonhuman primate and seems to have a substantial pharmacokinetic advantage over systemic administration for meningeal tumors.

Plasma and cerebrospinal fluid pharmacokinetics of 9-aminocamptothecin (9-AC), irinotecan (CPT-11), and SN-38 in nonhuman primates.

PURPOSE: The plasma and cerebrospinal fluid (CSF) pharmacokinetics of the camptothecin analogs, 9-aminocamptothecin (9-AC) and irinotecan, were studied in a nonhuman primate model to determine their CSF penetration. METHODS: 9-AC, 0.2 mg/kg (4 mg/m2) or 0.5 mg/kg (10 mg/m2), was infused intravenously over 15 min and irinotecan, 4.8 mg/kg (96 mg/m2) or 11.6 mg/kg (225 mg/m2), was infused over 30 min. Plasma and CSF samples were obtained at frequent intervals over 24 h. Lactone and total drug forms of 9-AC, irinotecan, and the active metabolite of irinotecan, SN-38, were quantified by reverse-phase HPLC. RESULTS: 9-AC lactone had a clearance (CL) of 2.1 +/- 0.9 l/kg per h, a volume of distribution at steady state (Vd[ss]) of 1.6 +/- 0.7 l/kg and a half-life (t1/2) of 3.2 +/- 0.8 h. The lactone form of 9-AC accounted for 26 +/- 7% of the total drug in plasma. The CSF penetration of 9-AC lactone was limited. CSF 9-AC lactone concentration peaked 30 to 45 min after the dose at 11 to 21 nM (0.5 mg/kg dose), and the ratio of the areas under the CSF and plasma concentration-time curves (AUC(CSF):AUC[P]) was only 3.5 +/- 2.1%. For irinotecan, the CL was 3.4 +/- 0.4 l/kg per h, the Vd(ss) was 7.1 +/- 1.3 l/kg, and the t1/2 was 4.9 +/- 2.2 h. Plasma AUCs of the lactone form of SN-38 were only 2.0% to 2.4% of the AUCs of irinotecan lactone. The lactone form of irinotecan accounted for 26 +/- 5% of the total drug in plasma, and the lactone form of SN-38 accounted for 55 +/- 6% of the total SN-38 in plasma. The AUC(CSF):AUC(P) ratio for irinotecan lactone was 14 +/- 3%. SN-38 lactone and carboxylate could not be measured (< 1.0 nM) in CSF. The AUC(CSF):AUC(P) ratio for SN-38 lactone was estimated to be < or = 8%. CONCLUSION: Despite their structural similarity, the CSF penetration of 9-AC and SN-38 is substantially less than that of topotecan which we previously found to have an AUC(CSF):AUC(P) ratio of 32%.

Pharmacokinetics of dideoxypurine nucleoside analogs in plasma and cerebrospinal fluid of rhesus monkeys.

The pharmacokinetics of 2',3'-dideoxyadenosine (ddA), didanosine, 2',3'-dideoxyguanosine (ddG), and 6-halogenated prodrugs of ddG, 6-chloro-ddG and 6-iodo-ddG, in plasma and cerebrospinal fluid (CSF) were studied in a non-human primate model. ddA was rapidly and completely deaminated to didanosine, such that didanosine concentration profiles in plasma and CSF were identical following administration of ddA and didanosine. The mean clearance of didanosine was 0.50 liters/h/kg, the terminal half-life was 1.8 h, and the CSF-to-plasma ratio was 4.8%. The disposition of ddG was similar, with a clearance of 0.70 liters/h/kg and a half-life of 1.7 h. The adenosine deaminase-mediated conversion of the 6-halogenated-ddG prodrugs to ddG was rapid but incomplete (48% for 6-chloro-ddG and 29% for 6-iodo-ddG). The CSF-to-plasma ratios of ddG with equimolar doses of ddG, 6-chloro-ddG, and 6-iodo-ddG were 8.5, 24, and 17%, respectively, but the actual ddG exposures in CSF (area under the CSF concentration-time curve) were comparable for ddG (12.1 microM.h) and the 6-halogenated-ddG prodrugs (18.8 microM.h for 6-chloro-ddG, 9.3 microM.h for 6-iodo-ddG).6-Chloro-ddG was not detectable in plasma or CSF, and the CSF-to-plasma ratio of 6-iodo-ddG was 9.4%, so the higher CSF-to-plasma ratios of ddG with the administration of the 6-halogenated-ddG prodrugs does not appear to be the result of enhanced penetration of the prodrug and subsequent dehalogenation to ddG. The penetration of ddG into CSF exceeds that of didanosine and is enhanced by administration of the 6-halogenated prodrugs, although the mechanism of this enhanced penetration is unclear.

Effect of body position on ventricular CSF methotrexate concentration following intralumbar administration.

PURPOSE: Intralumbar methotrexate is one of the primary therapeutic modalities for the prevention and treatment of meningeal leukemia. However, methotrexate distribution to the ventricles is limited and highly variable following intralumbar dosing, and cytotoxic concentrations of methotrexate are not always achieved or sustained in the ventricular CSF. We used a nonhuman primate model to determine the effect of body position on the caudal distribution of an intralumbar dose of methotrexate. METHODS: Methotrexate (1.0 mg) was administered by intralumbar injection to four animals, which were then immediately placed either in an upright sitting position or in a prone position for 1 hour, then upright. Each animal served as its own control and was studied in each position on at least one occasion. RESULTS: The mean peak ventricular methotrexate concentration was 0.12 mumol/L (range, 0.091 to 0.20) in animals that were immediately placed upright, compared with 2.81 mumol/L (range, 0.21 to 8.9) in animals that remained prone for 1 hour. The mean area under the concentration-versus-time curves (AUC) was 0.51 mumol/L.h (range, 0.26 to 1.1) in the upright animals and 12.0 mumol/L.h (range, 0.9 to 35.4) in the prone animals. CONCLUSION: Maintaining a prone position for 1 hour after an intralumbar dose increased the peak methotrexate concentration and drug exposure in ventricular CSF. CSF drug distribution following intralumbar therapy can be influenced by body position after the injection.

Cerebral subarachnoid sampling of cerebrospinal fluid in the rhesus monkey.

A previously described rhesus monkey model with two intraventricular catheter systems was expanded to provide a means of studying drug concentrations of chemotherapeutic agents such as methotrexate (MTX) in the cerebral subarachnoid cerebrospinal fluid (CSF) following intrathecal drug injection. A continuous intraventricular infusion of MTX was started through the lateral ventricular catheter 44 h before surgical placement of the cerebral subarachnoid catheter to allow for steady-state concentrations to be attained throughout the subarachnoid space. Monkeys were anesthetized intramuscularly with ketamine hydrochloride (7 mg/kg) and xylazine (6 mg/kg) to allow placement of a temporary lumbar catheter for sampling of lumbar CSF Sodium thiopental (2.5%) was given intravenously if needed for intubation and anesthesia was maintained with isoflurane (0.5 to 1.5%) and oxygen during the surgical placement and sampling of the cerebral subarachnoid catheter. A midline incision was made over the frontal bone and a 3/8-inch trephine was used to expose the dura adjacent to the lateral midline. Following puncture of the dura with an 18-gauge spinal needle, the cerebral subarachnoid catheter (0.025-inch [i.d.] x 0.047-inch [o.d.] Silastic tube) was passed into the subarachnoid space for approximately 10 to 17 mm. The CSF from the cerebral subarachnoid catheter was collected by gravity flow Ventricular, lumbar subarachnoid, and cerebral subarachnoid CSF were collected concurrently at 44, 46, and 48 h after the start of MTX infusion. Mean ventricular, lumbar subarachnoid, and cerebral subarachnoid CSF methotrexate concentrations at steady state were 5.8, 1.0, and 1.5 mumol/L, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Cerebrospinal fluid pharmacokinetics and toxicology of intraventricular and intrathecal arabinosyl-5-azacytosine (fazarabine, NSC 281272) in the nonhuman primate.

Arabinosyl-5-azacytosine (AAC), a new nucleoside antimetabolite, is broadly active in preclinical tumor screening evaluations. To assess the potential for intrathecal use of this drug, we studied the toxicity and pharmacokinetics of intrathecal and intraventricular administration in nonhuman primates. Four adult male rhesus monkeys were given single 10 mg intrathecal (n = 1) or intraventricular (n = 3) doses of AAC to determine its acute toxicity and pharmacokinetic parameters. An additional 3 animals were given four weekly 10 mg intrathecal doses to assess the systemic and neurologic toxicity associated with chronic administration. Disappearance from the cerebrospinal fluid (CSF) was biexponential, and CSF clearance was 0.2 ml/min, which exceeds the rate of CSF bulk flow by 5-fold. The peak CSF concentration and area under the concentration x time curve achieved with the intraventricular administration of 10 mg were one hundred, and fifty fold greater, respectively, than those achieved after an intravenous dose of 200 mg/kg (1500-2400 mg) in prior experiments. No clinically evident neurotoxicity was observed in either the single or the weekly x 4 dose groups. A slight, transient CSF pleocytosis and increased CSF protein was observed. Systemic toxicity was limited to one animal in the weekly x 4 dose group who demonstrated a mild and transient decrease in his peripheral leukocyte count unassociated with a change in his hematocrit or platelet count. These studies in nonhuman primates demonstrate a clear pharmacokinetic advantage for intrathecal vs systemic administration of AAC. This is demonstrated by a 50-fold greater CSF drug exposure with an intrathecal or intraventricular dose 1/200th of that which can be given systemically.(ABSTRACT TRUNCATED AT 250 WORDS)

Prolongation of drug exposure in cerebrospinal fluid by encapsulation into DepoFoam.

Prolonged maintenance of a therapeutic drug concentration in the cerebrospinal fluid is required for optimal treatment of leptomeningeal leukemia or carcinomatosis with cell cycle-specific antimetabolites. The pharmacokinetics of 1-beta-D-arabinofuranosylcytosine (ara-C) encapsulated into DepoFoam (Depo/Ara-C) was studied in six rhesus monkeys after intrathecal injection into the lumbar sac. Following a single 2-mg dose, the Depo/Ara-C concentration decreased biexponentially with initial and terminal half-lives of 14.6 and 156 h, respectively. The free drug concentration remained above the reported minimal cytotoxic level of 0.1 micrograms/ml (0.4 microM) for more than 672 h (28 days). In contrast, the half-life of ara-C following an intralumbar bolus dose of unencapsulated drug in a single animal was 0.74 h. A single intrathecal injection of Depo/Ara-C can maintain a therapeutic drug concentration in the cerebrospinal fluid for a very prolonged period.

Pharmacokinetics of PEG-L-asparaginase and plasma and cerebrospinal fluid L-asparagine concentrations in the rhesus monkey.

The pharmacokinetics of the polyethylene glycol-conjugated form of the enzyme L-asparaginase and the depletion of L-asparagine from the plasma and cerebrospinal fluid (CSF) following an i.m. dose of 2500 IU/m2 PEG-L-asparaginase was studied in rhesus monkeys. PEG-L-asparaginase activity in plasma was detectable by 1 h after injection and maintained a plateau of approximately 4 IU/ml for more than 5 days. Subsequent elimination from plasma was monoexponential with a half-life of 6 +/- 1 days. Plasma L-asparagine concentrations fell from pretreatment levels of 14-47 microM to < 2 microM by 24 h after injection in all animals and remained undetectable for the duration of the 25-day observation period in four of six animals. In two animals, plasma L-asparagine became detectable when the PEG-L-asparaginase plasma concentration dropped below 0.1 IU/ml. Pretreatment CSF L-asparagine levels ranged from 4.7 to 13.6 microM and fell to < 0.25 microM by 48 h in five of six animals. CSF L-asparagine concentrations remained below 0.25 microM for 10-14 days in four animals. One animal had detectable CSF L-asparagine concentrations within 24 h and another had detectable concentrations within 1 week of drug administration despite a plasma PEG-L-asparaginase activity profile that did not differ from that of the other animals. These observations may be useful in the design of clinical trials with PEG-L-asparaginase in which correlations among PEG-L-asparaginase pharmacokinetics, depletion of L-asparagine, and clinical outcome should be sought.

Methotrexate pharmacokinetics following administration of recombinant carboxypeptidase-G2 in rhesus monkeys.

PURPOSE: Carboxypeptidase-G2 (CPDG2) is a bacterial enzyme that rapidly hydrolyzes methotrexate (MTX) into inactive metabolites. As an alternative form of rescue after high-dose MTX (HDMTX), CPDG2 has more potential advantages than standard leucovorin (LV) rescue. In this study, the plasma pharmacokinetics of MTX with and without CPDG2 were evaluated in adult rhesus monkeys. MATERIALS AND METHODS: The plasma pharmacokinetics of MTX were determined in groups of animals that had received a 300-mg/m2 loading dose of MTX followed by a 60-mg/m2/h infusion during an 18-hour period. One group received CPDG2 at the end of the infusion, and the other group served as a control. Two additional animals with high titers of anti-CPDG2 antibody also were studied. RESULTS: During infusion, the steady-state MTX plasma concentration was 11.3 +/- 4.8 mumol/L. Without CPDG2, the postinfusion plasma MTX concentration remained above 0.1 mumol/L for more than 6 hours. After the administration of 50 U/kg of CPDG2, plasma MTX concentrations decreased to nontoxic levels (less than 0.05 mumol/L) within 30 minutes. The initial half-life (t1/2 alpha) of MTX decreased from 5.8 +/- 2.1 minutes to 0.7 +/- 0.02 minutes after enzyme administration. The postinfusion area under the plasma concentration time curve of MTX was 301 +/- 171 mumol/L/min without CPDG2 compared with 19.6 +/- 6.1 mumol/L/min with CPDG2. The immunogenicity studies performed indicated that although animals developed anti-CPDG2 antibodies, none of them manifested allergic symptoms. The effectiveness of CPDG2 was diminished but not eliminated in animals with high titers of anti-CPDG2 antibody. CONCLUSIONS: CPDG2 is capable of rapidly decreasing plasma MTX concentrations to nontoxic levels. The administration of CPDG2 seems safe, well tolerated, and it may be useful as an alternative to LV rescue.

Intrathecal 5-fluorouracil in the rhesus monkey.

Because meningeal spread of both leukemia and solid tumors remains a difficult therapeutic problem, there is a compelling need to develop new agents for intrathecal administration. 5-Fluorouracil (5FU), an active anticancer agent, penetrates into the central nervous system to some degree following intravenous dosing. Significant systemic toxicity, however, is associated with this route of administration. Therefore, the pharmacokinetic behavior of 5FU following its intrathecal administration was studied in a rhesus monkey model. After a 10-mg intraventricular dose, the disappearance of the drug from ventricular cerebrospinal fluid was monoexponential, the half-life being 51 min; the area under the concentration-time curve (AUC) being greater than 18 mM h-1; and the peak ventricular 5FU concentrations ranging between 10 and 15 mM. After a 1-mg intralumbar dose, the AUC was 1235 microM h-1. No toxicity was observed following intraventricular administration of 5FU. After intralumbar administration of either a 10-mg or a 1-mg dose, however, local toxicity was observed in the lumbar spinal cord. These findings suggest that intrathecal administration of 5FU is not presently a feasible means of achieving cytotoxic cerebrospinal fluid concentrations.

Pharmacokinetics of pyrazoloacridine in the rhesus monkey.

Pyrazoloacridine is a rationally synthesized acridine derivative with in vitro activity against solid tumor cell lines, noncycling and hypoxic cells, and tumor cell lines that exhibit the multidrug resistance phenotype. The pharmacokinetic behavior of pyrazoloacridine after a 1- or 24-h i.v. infusion was studied in 5 rhesus monkeys that received a total of 10 courses of pyrazoloacridine at 300 or 600 mg/m2. Pyrazoloacridine levels in plasma and cerebrospinal fluid were measured by high-pressure liquid chromatography. For 1-h infusions, the plasma disappearance was biexponential with a t 1/2 alpha of 31 min and t 1/2 beta of 11 h. The mean volume of distribution at steady state was 1380 liters/m2. The clearance was 1660 ml/min/m2. For the 300 mg/m2 dose, the mean area under the concentration-time curve was 759 microM.min, and the mean peak concentration was 1.3 microM. For the 600 mg/m2 dose, the area under the concentration-time curve was 1330 microM.min, and the peak concentration was 2.5 microM. The steady-state plasma concentrations during the 24-h continuous infusions were 0.27 microM for the 300 mg/m2 dose and 0.45 microM for the 600 mg/m2 dose. The mean clearance calculated from these steady-state concentrations was 2420 ml/min/m2. Cerebrospinal fluid levels were less than 0.1 microM for all doses and schedules. There was no evidence of toxicity at any dose or schedule. These results contrast strikingly with those obtained in mice and dogs in which, despite a more rapid clearance of pyrazoloacridine, significant toxicities were observed at doses that were nontoxic in the monkey. These interspecies differences in the pharmacokinetic and pharmacodynamic behavior of pyrazoloacridine have important implications for the design of Phase I trials in humans.