Inverse targeting of peritoneal tumors: selective alteration of the disposition of methotrexate through the use of anti-methotrexate antibodies and antibody fragments

Use of Payload Binding Selectivity Enhancers to Improve Therapeutic Index of Maytansinoid-Antibody-Drug Conjugates

Systemic exposure to released cytotoxic payload contributes to the dose-limiting off-target toxicities of anticancer antibody-drug conjugates (ADC). In this work, we present an "inverse targeting" strategy to optimize the therapeutic selectivity of maytansinoid-conjugated ADCs. Several anti-maytansinoid sdAbs were generated via phage-display technology with binding IC50 values between 10 and 60 nmol/L. Co-incubation of DM4 with the anti-maytansinoid sdAbs shifted the IC50 value of DM4 up to 250-fold. Tolerability and efficacy of 7E7-DM4 ADC, an anti-CD123 DM4-conjugated ADC, were assessed in healthy and in tumor-bearing mice, with and without co-administration of an anti-DM4 sdAb. Co-administration with anti-DM4 sdAb reduced 7E7-DM4-induced weight loss, where the mean values of percentage weight loss at nadir for mice receiving ADC+saline and ADC+sdAb were 7.9% ± 3% and 3.8% ± 1.3% (P < 0.05). In tumor-bearing mice, co-administration of the anti-maytansinoid sdAb did not negatively affect the efficacy of 7E7-DM4 on tumor growth or survival following dosing of the ADC at 1 mg/kg (P = 0.49) or at 10 mg/kg (P = 0.9). Administration of 7E7-DM4 at 100 mg/kg led to dramatic weight loss, with 80% of treated mice succumbing to toxicity before the appearance of mortality relating to tumor growth in control mice. However, all mice receiving co-dosing of 100 mg/kg 7E7-DM4 with anti-DM4 sdAb were able to tolerate the treatment, which enabled reduction in tumor volume to undetectable levels and to dramatic improvements in survival. In summary, we have demonstrated the utility and feasibility of the application of anti-payload antibody fragments for inverse targeting to improve the selectivity and efficacy of anticancer ADC therapy.

Mechanisms of ADC Toxicity and Strategies to Increase ADC Tolerability

Anti-cancer antibody-drug conjugates (ADCs) aim to expand the therapeutic index of traditional chemotherapy by employing the targeting specificity of monoclonal antibodies (mAbs) to increase the efficiency of the delivery of potent cytotoxic agents to malignant cells. In the past three years, the number of ADCs approved by the Food and Drug Administration (FDA) has tripled. Although several ADCs have demonstrated sufficient efficacy and safety to warrant FDA approval, the clinical use of all ADCs leads to substantial toxicity in treated patients, and many ADCs have failed during clinical development due to their unacceptable toxicity profiles. Analysis of the clinical data has demonstrated that dose-limiting toxicities (DLTs) are often shared by different ADCs that deliver the same cytotoxic payload, independent of the antigen that is targeted and/or the type of cancer that is treated. DLTs are commonly associated with cells and tissues that do not express the targeted antigen (i.e., off-target toxicity), and often limit ADC dosage to levels below those required for optimal anti-cancer effects. In this manuscript, we review the fundamental mechanisms contributing to ADC toxicity, we summarize common ADC treatment-related adverse events, and we discuss several approaches to mitigating ADC toxicity.

Unraveling the pharmacokinetic interaction of ticagrelor and MEDI2452 (Ticagrelor antidote) by mathematical modeling

The investigational ticagrelor‐neutralizing antibody fragment, MEDI2452, is developed to rapidly and specifically reverse the antiplatelet effects of ticagrelor. However, the dynamic interaction of ticagrelor, the ticagrelor active metabolite (TAM), and MEDI2452, makes pharmacokinetic (PK) analysis nontrivial and mathematical modeling becomes essential to unravel the complex behavior of this system. We propose a mechanistic PK model, including a special observation model for post‐sampling equilibration, which is validated and refined using mouse in vivo data from four studies of combined ticagrelor‐MEDI2452 treatment. Model predictions of free ticagrelor and TAM plasma concentrations are subsequently used to drive a pharmacodynamic (PD) model that successfully describes platelet aggregation data. Furthermore, the model indicates that MEDI2452‐bound ticagrelor is primarily eliminated together with MEDI2452 in the kidneys, and not recycled to the plasma, thereby providing a possible scenario for the extrapolation to humans. We anticipate the modeling work to improve PK and PD understanding, experimental design, and translational confidence.

Scale-up of a physiologically-based pharmacokinetic model to predict the disposition of monoclonal antibodies in monkeys

Preclinical assessment of monoclonal antibody (mAb) disposition during drug development often includes investigations in non-human primate models. In many cases, mAb exhibit non-linear disposition that relates to mAb-target binding [i.e., target-mediated disposition (TMD)]. The goal of this work was to develop a physiologically-based pharmacokinetic (PBPK) model to predict non-linear mAb disposition in plasma and in tissues in monkeys. Physiological parameters for monkeys were collected from several sources, and plasma data for several mAbs associated with linear pharmacokinetics were digitized from prior literature reports. The digitized data displayed great variability; therefore, parameters describing inter-antibody variability in the rates of pinocytosis and convection were estimated. For prediction of the disposition of individual antibodies, we incorporated tissue concentrations of target proteins, where concentrations were estimated based on categorical immunohistochemistry scores, and with assumed localization of target within the interstitial space of each organ. Kinetics of target-mAb binding and target turnover, in the presence or absence of mAb, were implemented. The model was then employed to predict concentration versus time data, via Monte Carlo simulation, for two mAb that have been shown to exhibit TMD (2F8 and tocilizumab). Model predictions, performed a priori with no parameter fitting, were found to provide good prediction of dose-dependencies in plasma clearance, the areas under plasma concentration versu time curves, and the time-course of plasma concentration data. This PBPK model may find utility in predicting plasma and tissue concentration versus time data and, potentially, the time-course of receptor occupancy (i.e., mAb-target binding) to support the design and interpretation of preclinical pharmacokinetic-pharmacodynamic investigations in non-human primates.

PK/TD modeling for prediction of the effects of 8C2, an anti-topotecan mAb, on topotecan-induced toxicity in mice

To facilitate the development of an inverse targeting strategy, where anti-topotecan antibodies are administered to prevent systemic toxicity following intraperitoneal topotecan, a pharmacokinetic/toxicodynamic (PK/TD) model was developed and evaluated. The pharmacokinetics of 8C2, a monoclonal anti-topotecan antibody, were assessed following IV and SC administration, and the data were characterized using a two compartmental model with nonlinear absorption and elimination. A hybrid PK model was constructed by combining a PBPK model for topotecan with the two-compartment model for 8C2, and the model was employed to predict the disposition of topotecan, 8C2, and the topotecan-8C2 complex. The model was linked to a toxicodynamic model for topotecan-induced weight-loss, and simulations were conducted to predict the effects of 8C2 on the toxicity of topotecan in mice. Increasing the molar dose ratio of 8C2 to topotecan resulted in a dose-dependent decrease in the unbound (i.e., not bound to 8C2) topotecan exposure in plasma (AUCf) and a decrease in the extent of topotecan-induced weight-loss. Consistent with model predictions, toxicodynamic experiments showed substantial reduction in the percent nadir weight loss observed with 30 mg/kg IP topotecan after co-administration of 8C2 (20 ± 8% vs. 10 ± 8%). The investigation supports the use of anti-topotecan mAb to reduce the systemic toxicity of IP topotecan chemotherapy.

Predicting the effects of 8C2, a monoclonal anti-topotecan antibody, on plasma and tissue disposition of topotecan

We are investigating an inverse targeting strategy to reduce the dose limiting systemic toxicities resultant from intraperitoneal administration of topotecan, a model chemotherapeutic drug. This approach utilizes systemic co-administration of anti-topotecan antibodies to alter the plasma and tissue disposition kinetics of topotecan. To better predict the effects of 8C2, a high affinity anti-topotecan monoclonal antibody, on the pharmacokinetics of topotecan, two mathematical models have been developed and evaluated. Model 1 is a hybrid physiologically based pharmacokinetic (PBPK) model that was created by merging a PBPK model for topotecan with a simple two compartment model of 8C2 pharmacokinetics. Model 2 is a comprehensive PBPK model developed by merging a PBPK model for IgG with a PBPK model for topotecan. To help validate the simulation results from both the models, a tissue distribution experiment was conducted, in which topotecan and 8C2 were co-administered in mice. Experimental and simulated data were compared by calculating the median percent prediction error (%PE) for all tissues. For both models, the median %PE values for all the tissues were less than 100 %, indicating that the predicted values were, on average, less than twofold the observed plasma and tissue topotecan concentrations values. In general model 2 was found to be more predictive of the data set than model 1, as the overall median %PE value for model 2 (%PE = 63) was less than model 1 (%PE = 73).

Physiologically based pharmacokinetic model for topotecan in mice

Topotecan is a chemotherapeutic agent of choice for the second-line treatment of recurrent ovarian cancer. In this article, we have developed a physiologically based pharmacokinetic model to characterize and predict topotecan concentrations in mouse plasma and tissues. Single intravenous (IV) doses (5, 10 and 30 mg/kg) of topotecan were administered to male Swiss Webster mice, with plasma and tissue samples collected over 24 h, and with sample analysis by high performance liquid chromatography. Topotecan disposition in the lungs, heart, muscle, skin, spleen, gut, liver, brain and adipose was described by perfusion rate-limited compartments, whereas the testes and intraperitoneal (IP) fluid were described with permeability rate-limited compartments. The kidneys were modeled as a permeability rate-limited compartment with nonlinear efflux. The model included enterohepatic recycling of topotecan, with re-absorption of drug secreted in the bile and nonlinear bioavailability. Topotecan demonstrated dose-dependent, nonlinear pharmacokinetics and its elimination was described by nonlinear clearance from the liver and a parallel nonlinear and linear clearance from the kidneys. Mean tissue-to-plasma partition coefficients ranged from 0.123 (brain) to 55.3 (kidney). The model adequately characterized topotecan pharmacokinetics in plasma and tissue for all three doses. Additionally, the model provided good prediction of topotecan pharmacokinetics from several external data sets, including prediction of topotecan tissue pharmacokinetics following administration of 1 or 20 mg/kg IV, and prediction of plasma pharmacokinetics following doses of 1, 1.25, 15, 20 and 80 mg/kg IV and 20 mg/kg IP.

Properties of a general PK/PD model of antibody-ligand interactions for therapeutic antibodies that bind to soluble endogenous targets

Antibodies that target endogenous soluble ligands are an important class of biotherapeutic agents. While much focus has been placed on characterization of antibody pharmacokinetics, less emphasis has been given to characterization of antibody effects on their soluble targets. We describe here the properties of a generalized mechanism-based PK/PD model used to characterize the in vivo interaction of an antibody and an endogenous soluble ligand. The assumptions and properties of the model are explored, and situations are described when deviations from the basic assumptions may be necessary. This model is most useful for in vivo situations where both antibody and ligand levels are available following drug administration. For a given antibody exposure, the extent and duration of suppression of free ligand is impacted by the apparent affinity of the interaction, as well as by the rate of ligand turnover. The applicability of the general equilibrium model of in vivo antibody-ligand interaction is demonstrated with an anti-Aß antibody.

Mathematical modeling of topotecan pharmacokinetics and toxicodynamics in mice

The objective of this study was to investigate the pharmacokinetics and toxicodynamics of topotecan (TPT) in mice and to develop an integrated pharmacokinetic/toxicodynamic (PK/TD) model to characterize the relationship between the time course of TPT disposition and the time course of TPT-induced toxicity. TPT was administered to groups of 3-5 mice via i.v. bolus injection, i.p. bolus injection, and by i.p. infusion over 24, 72 and 168 h. Body weight was monitored to assess TPT-induced toxicity, and serial blood samples were collected and analyzed via HPLC to assess TPT pharmacokinetics. We found that TPT-induced toxicity increased dose-dependently for each mode of dosing investigated. The time course of topotecan-induced body weight-loss was delayed relative to the time course of topotecan disposition; nadir body weight was observed as late as 6 days following i.p. bolus dosing, and 3-5 days following termination of i.p. infusion. TPT exhibited non-linear disposition, which was well-characterized through the use of a two-compartment model with saturable elimination from the central compartment. Toxicodynamic data were characterized with an integrated PK/TD model that incorporated an indirect-effect model and four transit compartments to describe transduction events associated with TPT-induced toxicity. This model will be used to support the development of an inverse-targeting strategy that aims to enhance topotecan safety and efficacy.

Investigation of antibody-coated liposomes as a new treatment for immune thrombocytopenia

Immune thrombocytopenia (ITP) is an autoimmune disease that is mediated by anti-platelet antibodies. It is believed that anti-platelet antibody-opsonized platelets are eliminated through Fcgamma receptor-mediated and complement-mediated phagocytosis by macrophages of the reticuloendothelial system (RES). Polyclonal pooled immunoglobulin with high titer for the D-antigen of erythrocytes (i.e., anti-D) has been successfully used to ameliorate ITP. Based on the pathogenesis of ITP and based on the successful application of anti-D for the treatment of ITP, we hypothesized that antibody-coated liposomes may be used to inhibit Fcgamma receptor-mediated and complement-mediated phagocytosis, thereby increasing platelet counts in ITP. To test this hypothesis, we have developed a liposome preparation that is coated with a model monoclonal IgG1 antibody. Antibody-coated liposomes were found to inhibit complement deposition and macrophage phagocytosis in vitro. Furthermore, antibody-coated liposomes were also found to attenuate thrombocytopenia in a rat model of ITP, in a dose-dependent manner. The results suggest that antibody-coated liposomes may be used as 'decoy particles' to competitively inhibit the destruction of antibody-coated platelets; thus, antibody-coated liposomes may have value in the treatment of ITP.

Application of Anti-Methotrexate Fab Fragments for the Optimization of Intraperitoneal Methotrexate Therapy in a Murine Model of Peritoneal Cancer

Anti-drug antibodies may be used to impart regio-specific alterations in drug disposition, potentially enhancing the therapeutic selectivity of intracavitary chemotherapy. In the present study, we tested the hypotheses that systemic therapy with anti-methotrexate antibodies would allow increases in the maximum tolerated dose of intraperitoneal methotrexate (MTX) and allow increases in the therapeutic efficacy of intraperitoneal MTX in a murine model of peritoneal cancer. Monoclonal anti-MTX Fab antibody fragments (AMF) were produced, purified, and characterized. AMF pharmacokinetics were determined following i.v. bolus injection (0.4 g/kg) and s.c. bolus injection (0.4, 0.8, 2.2 g/kg). MTX efficacy was investigated in mice bearing peritoneal sarcoma 180 tumors, following administration of MTX via 72 h i.p. infusion at 1.9, 2.8, 3.8 mg/kg, and following combination therapy of 7.5 or 10 mg/kg i.p. MTX (72 h infusion) and 4.2 g/kg s.c. AMF. The mean terminal half-life of AMF was found to be 10.9 +/- 3.3 h and was not dose-dependent, and s.c. bioavailability was 28% +/- 7% at 2.2 g/kg. In mice bearing peritoneal tumors, the maximally tolerated dose of i.p. MTX increased from 1.9 mg/kg (following i.p. MTX alone) to 10 mg/kg (with co-administration of s.c. AMF). Median survival times for saline-treated control animals and animals receiving i.p. MTX (1.9, 2.8, 3.8 mg/kg) were 9, 12, 10, and 7 days, respectively. However, for animals receiving combination therapy with i.p. MTX 7.5 or 10 mg/kg and 4.2 g/kg s.c. AMF, median survival time increased to 17 and 14 days, respectively. As such, the present data suggest that systemic administration of AMF may allow increases in the maximally tolerated dose of i.p. MTX, and allow increases in the therapeutic efficacy of i.p. MTX chemotherapy of peritoneal tumors.

High-performance liquid chromatographic assay for the determination of total and free topotecan in the presence and absence of anti-topotecan antibodies in mouse plasma

A rapid and sensitive high-performance liquid chromatographic (HPLC) assay has been developed to allow determination of total (i.e. bound and unbound) and free (i.e. unbound) topotecan (TPT) in mouse plasma in the presence and absence of anti-TPT antibodies. The chromatographic analysis was carried out using reversed-phase isocratic elution with a Nova-Pak C18 column (3.9 mm x 150 mm, 4 microm) protected by a Nova-Pak C18 guard column (3.9 mm x 20 mm, 4 microm), where 10 mM KH(2)PO(4)-methanol-triethylamine (72:26:2 (v/v/v), pH 3.5) was used as the mobile phase. Topotecan was quantified with fluorescence detection using an excitation wavelength of 361 nm and an emission wavelength of 527 nm. The retention time for the internal standard, acridine, and TPT were 7.4 and 9.0 min, respectively. The lower limit of quantitation (LOQ) for TPT was determined as 0.02 ng in mouse plasma and mouse plasma ultrafiltrate, corresponding to a concentration of 1 ng/ml in 20 microl mouse plasma. The assay was shown to be linear over a concentration range of 1-500 ng/ml. The recoveries of free and total TPT from spiked mouse plasma were within 10% of theoretical values (assessed at 1, 20 and 500 ng/ml). The validated HPLC assay was applied to evaluate TPT pharmacokinetics following administration of TPT to Swiss Webster mice and to hyperimmunized and control BALB/c mice. The assay has been shown to be capable for measuring total and free TPT in mouse plasma with high sensitivity and will allow the testing of the effect of anti-TPT antibodies on the disposition of TPT.

Antibody pharmacokinetics and pharmacodynamics

The U.S. Food and Drug administration (FDA) has approved several polyclonal antibody preparations and at least 18 monoclonal antibody preparations (antibodies, antibody fragments, antibody fusion proteins, etc.). These drugs, which may be considered as a diverse group of therapeutic proteins, are associated with several interesting pharmacokinetic characteristics. Saturable binding with target antigen may influence antibody disposition, potentially leading to nonlinear distribution and elimination. Independent of antigen interaction, concentration-dependent elimination may be expected for IgG antibodies, due to the influence of the Brambell receptor, FcRn, which protects IgG from catabolism. Antibody administration may induce the development of an endogenous antibody response, which may alter the pharmacokinetics of the therapeutic antibody. Additionally, the pharmacodynamics of antibodies are also complex; these drugs may be used for a wide array of therapeutic applications, and effects may be achieved by a variety of mechanisms. This article provides an overview of many of the complexities associated with antibody pharmacokinetics and pharmacodynamics.

Development and Validation of Enzyme‐Linked Immunosorbent Assays for Quantification of Anti‐Methotrexate IgG and Fab in Mouse and Rat Plasma

This laboratory is investigating the use of anti-methotrexate IgG (AMI) and anti-methotrexate Fab fragments (AMF) within an inverse targeting strategy that is designed to enhance the pharmacokinetic selectivity of intraperitoneal (i.p.) chemotherapy. The goal of this study was to develop enzyme-linked immunosorbent assays (ELISAs) to determine concentrations of AMI and AMF in mouse and rat plasma. An antigen-specific ELISA was developed for AMI and AMF in mouse and rat plasma. The assay was validated with respect to precision and accuracy by evaluating the recovery of AMI and AMF from mouse and rat plasma samples. Preliminary pharmacokinetic studies of AMI and AMF were performed in Sprague-Dawley rats and Swiss Webster mice. The animals were instrumented with a jugular vein cannula and administered AMI or AMF, 15 mg kg(-1) via the cannula. Plasma samples were taken at various time points and analyzed using the ELISA, and the observed concentration vs. time profiles were subjected to non-compartmental pharmacokinetic analyses. Standard curves for the ELISAs were found to be linear over concentration ranges of 0-250 and 0-350 ng mL(-1) for AMI and AMF, respectively. Intra-assay and inter-assay recovery of AMI and AMF from plasma samples were found to be within 15% of theoretical values. Preliminary pharmacokinetic investigations of AMI allowed estimation of AMI clearance to be 0.017 mL kg(-1) min(-1) in the rat and 0.043 mL kg(-1) min(-1) in the mouse. AMF clearance was estimated to be 0.038 and 1.93 mL kg(-1) min(-1) in the mouse and rat, respectively. In conclusion, ELISAs have been developed and validated for quantitation of AMI and AMF in rat and mouse plasma. The assays will allow further investigations of AMI and AMF pharmacokinetics.

Application of pharmacokinetic-pharmacodynamic modeling to predict the kinetic and dynamic effects of anti-methotrexate antibodies in mice

We have shown that intravenous (i.v.) administration of anti-methotrexate (MTX) antibodies (AMAb) reduces the systemic exposure of intraperitoneal (i.p.) MTX therapy, and we have proposed that AMAb effects on MTX systemic exposure would allow a reduction in MTX-induced systemic toxicity (i.e., producing a desirable antagonistic effect). However, many literature reports have shown that anti-toxin antibodies occasionally demonstrate unexpected agonist-like activity, increasing the extent of toxicity induced by their ligand. In this report, we have utilized a pharmacokinetic-pharmacodynamic (PKPD) model to predict the potential of AMAb to increase or decrease the magnitude of MTX-induced body weight loss in mice. Simulations predicted that both anti-MTX immunoglobulin G (AMI) and anti-MTX Fab fragments (AMF) would lead to increases or decreases in MTX toxicity, with effects dependent on the dosing protocol used. Based on the computer simulations, two protocols were selected for in vivo evaluation of predicted agonistic or antagonistic effects. Murine monoclonal AMI and AMF were produced, purified, and characterized. Agonistic effects were tested after 24-h infusion of i.p. MTX (10 mg/kg) and i.v. administration of an equimolar dose of AMI. Antagonistic effects were tested after 72-h infusion of i.p. MTX (5 mg/kg) and i.v. infusion of an equimolar dose of AMF. Consistent with model predictions of agonist-like activity, the 24-h AMI protocol led to significantly increased animal mortality (all animals died, p < 0.005) and mean nadir weight loss (p < 0.005). Also consistent with the predictions of the PKPD model, the 72-h AMF protocol significantly decreased animal mortality and mean nadir body weight loss (p < 0.01). Thus, these studies demonstrate that agonistic and antagonistic effects of anti-toxin antibodies may be predicted through the use of an integrated PKPD model.

Pharmacokinetic-pharmacodynamic modeling of methotrexate-induced toxicity in mice

The prediction of chemotherapeutic efficacy is complicated by "protocol dependencies" in dose-effect and dose-toxicity relationships. It has been proposed that pharmacokinetic-pharmacodynamic mathematical models may allow characterization of chemotherapeutic protocol dependencies, and may facilitate the prediction of chemotherapeutic efficacy; however, few demonstrations exist in the literature. The present study examines the pharmacokinetics and toxicodynamics of methotrexate (MTX), a commonly used anticancer agent, after intraperitoneal (i.p.) administration to mice. MTX was administered via bolus or infusion (24, 72, and 168 h), at doses of 2.5-1000 mg/kg. MTX plasma and peritoneal pharmacokinetics were characterized through standard noncompartmental and compartmental techniques. Body weight loss was used as a measure of MTX-induced toxicity. We found that MTX pharmacokinetics were independent of dose (over a range of 3-600 mg/kg) and independent of dosing mode (i.e., i.p. bolus vs. i.p. infusion). However, MTX-induced toxicity was shown to be highly dependent on the dosing protocol used. For example, the maximally tolerated dose (i.e., the dose related to a mean body weight loss of 10%) was 200-fold greater after bolus administration relative to that observed for 72-h infusion (760 mg/kg vs. 3.8 mg/kg). This profound protocol dependence in the relationship between MTX-induced toxicity and MTX exposure was characterized through the use of a time-dissociated pharmacokinetic-pharmacodynamic model (median prediction error: 3.9%).

Highly sensitive high-performance liquid chromatographic assay for methotrexate in the presence and absence of anti-methotrexate antibody fragments in rat and mouse plasma

Recently, Balthasar and Fung have proposed that anti-methotrexate antibody fragments may be employed to enhance the selectivity of intraperitoneal methotrexate (MTX) therapy. This current work presents a sensitive high-performance liquid chromatographic method for measuring plasma concentrations of total (i.e., bound and unbound) MTX and free (unbound) MTX in rat and mouse plasma, in the presence or absence of therapeutic anti-MTX antibody fragments. The assay involves pre-column derivatization of MTX by sodium hydrosulfite to 2,4-diamino-6-methylpteridine. The limit of quantitation for MTX by this assay was 1.25 ng in rat plasma, mouse plasma and mouse plasma ultrafiltrate, which corresponds to a concentration of 25 ng/ml for a 50 microl sample. The limit of quantitation was found to be 2.5 ng in rat plasma ultrafiltrate (i.e., 50 ng/ml in 50 microl rat plasma ultrafiltrate). The method was shown to be quite accurate, as the mean assayed concentration of quality control samples was within 10% of theoretical values. We have applied the method to the investigation of MTX pharmacokinetics in mice and rats, following the administration of MTX alone or following simultaneous administration of MTX and anti-MTX Fab fragments. The method has been shown to be suitable for the assay of total and free methotrexate in the plasma of these species and will enable the testing of pharmacokinetic hypotheses regarding the influence of anti-MTX Fab fragments on the disposition of MTX.