Pharmacokinetics of methotrexate in solid tumors

Combining microenvironment normalization strategies to improve cancer immunotherapy

Advances in immunotherapy have revolutionized the treatment of multiple cancers. Unfortunately, tumors usually have impaired blood perfusion, which limits the delivery of therapeutics and cytotoxic immune cells to tumors and also results in hypoxia-a hallmark of the abnormal tumor microenvironment (TME)-that causes immunosuppression. We proposed that normalization of TME using antiangiogenic drugs and/or mechanotherapeutics can overcome these challenges. Recently, immunotherapy with checkpoint blockers was shown to effectively induce vascular normalization in some types of cancer. Although these therapeutic approaches have been used in combination in preclinical and clinical studies, their combined effects on TME are not fully understood. To identify strategies for improved immunotherapy, we have developed a mathematical framework that incorporates complex interactions among various types of cancer cells, immune cells, stroma, angiogenic molecules, and the vasculature. Model predictions were compared with the data from five previously reported experimental studies. We found that low doses of antiangiogenic treatment improve immunotherapy when the two treatments are administered sequentially, but that high doses are less efficacious because of excessive vessel pruning and hypoxia. Stroma normalization can further increase the efficacy of immunotherapy, and the benefit is additive when combined with vascular normalization. We conclude that vessel functionality dictates the efficacy of immunotherapy, and thus increased tumor perfusion should be investigated as a predictive biomarker of response to immunotherapy.

Reengineering the Physical Microenvironment of Tumors to Improve Drug Delivery and Efficacy: From Mathematical Modeling to Bench to Bedside

Physical forces have a crucial role in tumor progression and cancer treatment. The application of principles of engineering and physical sciences to oncology has provided powerful insights into the mechanisms by which these forces affect tumor progression and confer resistance to delivery and efficacy of molecular, nano-, cellular, and immuno-medicines. Here, we discuss the mechanics of the solid and fluid components of a tumor, with a focus on how they impede the transport of therapeutic agents and create an abnormal tumor microenvironment (TME) that fuels tumor progression and treatment resistance. We also present strategies to reengineer the TME by normalizing the tumor vasculature and the extracellular matrix (ECM) to improve cancer treatment. Finally, we summarize various mathematical models that have provided insights into the physical barriers to cancer treatment and revealed new strategies to overcome these barriers.

Overview of Methods for Overcoming Hindrance to Drug Delivery to Tumors, with Special Attention to Tumor Interstitial Fluid

Every drug used to treat cancer (chemotherapeutics, immunological, monoclonal antibodies, nanoparticles, radionuclides) must reach the targeted cells through the tumor environment at adequate concentrations, in order to exert their cell-killing effects. For any of these agents to reach the goal cells, they must overcome a number of impediments created by the tumor microenvironment (TME), beginning with tumor interstitial fluid pressure (TIFP), and a multifactorial increase in composition of the extracellular matrix (ECM). A primary modifier of TME is hypoxia, which increases the production of growth factors, such as vascular endothelial growth factor and platelet-derived growth factor. These growth factors released by both tumor cells and bone marrow recruited myeloid cells form abnormal vasculature characterized by vessels that are tortuous and more permeable. Increased leakiness combined with increased inflammatory byproducts accumulates fluid within the tumor mass (tumor interstitial fluid), ultimately creating an increased pressure (TIFP). Fibroblasts are also up-regulated by the TME, and deposit fibers that further augment the density of the ECM, thus, further worsening the TIFP. Increased TIFP with the ECM are the major obstacles to adequate drug delivery. By decreasing TIFP and ECM density, we can expect an associated rise in drug concentration within the tumor itself. In this overview, we will describe all the methods (drugs, nutraceuticals, and physical methods of treatment) able to lower TIFP and to modify ECM used for increasing drug concentration within the tumor tissue.

Normalizing tumor microenvironment to treat cancer: bench to bedside to biomarkers

For almost four decades, my work has focused on one challenge: improving the delivery and efficacy of anticancer therapeutics. Working on the hypothesis that the abnormal tumor microenvironment-characterized by hypoxia and high interstitial fluid pressure--fuels tumor progression and treatment resistance, we developed an array of sophisticated imaging technologies and animal models as well as mathematic models to unravel the complex biology of tumors. Using these tools, we demonstrated that the blood and lymphatic vasculature, fibroblasts, immune cells, and extracellular matrix associated with tumors are abnormal, which together create a hostile tumor microenvironment. We next hypothesized that agents that induce normalization of the microenvironment can improve treatment outcome. Indeed, we demonstrated that judicious use of antiangiogenic agents--originally designed to starve tumors--could transiently normalize tumor vasculature, alleviate hypoxia, increase delivery of drugs and antitumor immune cells, and improve the outcome of various therapies. Our trials of antiangiogenics in patients with newly diagnosed and recurrent glioblastoma supported this concept. They revealed that patients whose tumor blood perfusion increased in response to cediranib survived 6 to 9 months longer than those whose blood perfusion did not increase. The normalization hypothesis also opened doors to treating various nonmalignant diseases characterized by abnormal vasculature, such as neurofibromatosis type 2. More recently, we discovered that antifibrosis drugs capable of normalizing the tumor microenvironment can improve the delivery and efficacy of nano- and molecular medicines. Our current efforts are directed at identifying predictive biomarkers and more-effective strategies to normalize the tumor microenvironment for enhancing anticancer therapies.

Delivery of molecular and cellular medicine to solid tumors

To reach cancer cells in a tumor, a blood-borne therapeutic molecule or cell must make its way into the blood vessels of the tumor and across the vessel wall into the interstitium, and finally migrate through the interstitium. Unfortunately, tumors often develop in ways that hinder each of these steps. Our research goals are to analyze each of these steps experimentally and theoretically, and then integrate the resulting information in a unified theoretical framework. This paradigm of analysis and synthesis has allowed us to obtain a better understanding of physiological barriers in solid tumors, and to develop novel strategies to exploit and/or to overcome these barriers for improved cancer detection and treatment.

Transport of molecules, particles, and cells in solid tumors

Extraordinary advances in molecular biology and biotechnology have led to the development of a vast number of therapeutic anti-cancer agents. To reach cancer cells in a tumor, a blood-borne therapeutic molecule, particle, or cell must make its way into the blood vessels of the tumor and across the vessel wall into the interstitium, which it then must migrate through. Unfortunately, tumors often develop in ways that hinder these steps. The goal of research in this area is to analyze each of these steps experimentally and theoretically and integrate the resulting information into a unified theoretical framework. This paradigm of analysis and synthesis has fostered a better understanding of physiological barriers in solid tumors and aided in the development of novel strategies to exploit and/or overcome these barriers for improved cancer detection and treatment.

Delivery of molecular medicine to solid tumors: lessons from in vivo imaging of gene expression and function

Extraordinary advances in molecular medicine and biotechnology have led to the development of a vast number of anti-cancer therapeutic agents. To reach cancer cells in a tumor, a blood-borne therapeutic molecule, particle or cell must make its way into the blood vessels of the tumor and across the vessel wall into the interstitium and finally migrate through the interstitium. Unfortunately, tumors often develop in ways that hinder each of these steps. Our research goals are to analyze each of these steps experimentally and theoretically and then integrate the resulting information in a unified theoretical framework. This paradigm of analysis and synthesis has allowed us to obtain a better understanding of microcirculatory barriers in solid tumors and to develop novel strategies to exploit and/or to overcome these barriers for improved cancer detection and treatment.

Delivery of molecular and cellular medicine to solid tumors

To reach cancer cells in a tumor, a blood-borne therapeutic molecule or cell must make its way into the blood vessels of the tumor and across the vessel wall into the interstitium, and finally migrate through the interstitium. Unfortunately, tumors often develop in ways that hinder each of these steps. Our research goals are to analyze each of these steps experimentally and theoretically, and then integrate the resulting information in a unified theoretical framework. This paradigm of analysis and synthesis has allowed us to obtain a better understanding of physiological barriers in solid tumors, and to develop novel strategies to exploit and/or to overcome these barriers for improved cancer detection and treatment.

Pharmacokinetics of methotrexate in the extracellular fluid of brain C6-glioma after intravenous infusion in rats

PURPOSE

Establishment of the pharmacokinetic profile of methotrexate (MTX) in the extracellular fluid (ECF) of a brain C6-glioma in rats.

METHODS

Serial collection of plasma samples and ECF dialysates after i.v. infusion of MTX (50 or 100 mg/kg) for 4 h. HPLC assay.

RESULTS

Histological studies revealed the presence of inflammation, edema, necrosis, and hemorrhage in most animals. In vivo recovery (reverse dialysis) was 10.8 +/- 5.3%. MTX concentrations in tumor ECF represented about 1-2% of the plasma concentrations. Rapid equilibration between MTX levels in brain tumor ECF and plasma. ECF concentrations almost reached steady-state by the end of the infusion (4 h), then decayed in parallel with those in plasma. Doubling of the dose did not modify MTX pharmacokinetic parameters (t1/2alpha, t1/2beta, MRT, fb, Vd, and CL(T)), except for a 1.7-fold increase of AUC(Plasma) and a 3.8-fold increase in AUC(ECF), which resulted in a 2.3-fold increase in penetration (AUC(ECF)/AUC(Plasma)). In spite of an important interindividual variability, a relationship between MTX concentrations in plasma and tumor ECF could be established from mean pharmacokinetic parameters.

CONCLUSIONS

High plasma concentrations promote the penetration of MTX into brain tissue. However, free MTX concentrations in tumor ECF remain difficult to predict consistently.

Delivery of molecular and cellular medicine to solid tumors

To reach cancer cells in a tumor, a blood-borne therapeutic molecule or cell must make its way into the blood vessels of the tumor and across the vessel wall into the interstitium, and finally migrate through the interstitium. Unfortunately, tumors often develop in ways that hinder each of these steps. Our research goals are to analyze each of these steps experimentally and theoretically, and then integrate the resulting information in a unified theoretical framework. This paradigm of analysis and synthesis has allowed us to obtain a better understanding of physiological barriers in solid tumors, and to develop novel strategies to exploit and/or to overcome these barriers for improved cancer detection and treatment.

Delivery of molecular and cellular medicine to solid tumors

To reach cancer cells in a tumor, a blood-borne therapeutic molecule or cell must make its way into the blood vessels of the tumor and across the vessel wall into the interstitium, and finally migrate through the interstitium. Unfortunately, tumors often develop in ways that hinder each of these steps. Our research goals are to analyze each of these steps experimentally and theoretically, and then integrate the resulting information in a unified theoretical framework. This paradigm of analysis and synthesis has allowed us to obtain a better understanding of physiological barriers in solid tumors, and to develop novel strategies to exploit and/or to overcome these barriers for improved cancer detection and treatment.

1995 Whitaker Lecture: delivery of molecules, particles, and cells to solid tumors

To reach cancer cells in a tumor, a blood-borne therapeutic agent must make its way into the blood vessels of the tumor and across the vessel wall into the interstitium, and finally migrate through the interstitium. Unfortunately, tumors often develop in ways that hinder each of these steps. Our research goals are to analyze each of these steps experimentally and theoretically, and then to integrate the resulting information in a unified theoretical framework. This paradigm of analysis and synthesis has allowed us to obtain a better understanding of physiological barriers in solid tumors, and to develop novel strategies to exploit and/or to overcome these barriers for improved cancer detection and treatment.

Haemodynamic and transport barriers to the treatment of solid tumours

The efficacy in cancer treatment of novel therapeutic agents such as monoclonal antibodies, cytokines and effector cells has been limited by their inability to reach their target in vivo in adequate quantities. Molecular and cellular biology of neoplastic cells alone has failed to explain the nonuniform uptake of these agents. This is not surprising since a solid tumour in vivo is not just a collection of cancer cells. In fact, it consists of two extracellular compartments: vascular and interstitial. Since no blood-borne molecule or cell can reach cancer cells without passing through these compartments, the vascular and interstitial physiology of tumours has received considerable attention in recent years. Three physiological factors responsible for the poor localization of macromolecules in tumours have been identified: (i) heterogeneous blood supply, (ii) elevated interstitial pressure, and (iii) large transport distances in the interstitium. The first factor limits the delivery of blood-borne agents to well-perfused regions of a tumour; the second factor reduces extravasation of fluid and macromolecules in the high interstitial pressure regions and also leads to an experimentally verifiable, radially outward convection in the tumour periphery which opposes the inward diffusion; and the third factor increases the time required for slowly moving macromolecules to reach distal regions of a tumour. Binding of the molecule to an antigen further lowers the effective diffusion rate by reducing the amount of mobile molecule. Although the effector cells are capable of active migration, peculiarities of the tumour vasculature and interstitium may also be responsible for poor delivery of lymphokine activated killer cells and tumour infiltrating lymphocytes in solid tumours. Due to micro- and macroscopic heterogeneities in tumours, the relative magnitude of each of these physiological barriers would vary from one location to another and from one day to the next in the same tumour, and from one tumour to another. If the genetically engineered macromolecules and effector cells, as well as low molecular weight cytotoxic agents, are to fulfill their clinical promise, strategies must be developed to overcome or exploit these barriers. Some of these strategies are discussed, and situations wherein these barriers may not be a problem are outlined. Finally, some therapies where the tumour vasculature of the interstitium may be a target are pointed out.

Vascular and interstitial barriers to delivery of therapeutic agents in tumors

The efficacy in cancer treatment of novel therapeutic agents such as monoclonal antibodies, cytokines and effector cells has been limited by their inability to reach their target in vivo in adequate quantities. Molecular and cellular biology of neoplastic cells alone has failed to explain the nonuniform uptake of these agents. This is not surprising since a solid tumor in vivo is not just a collection of cancer cells. In fact, it consists of two extracellular compartments: vascular and interstitial. Since no blood-borne molecule or cell can reach cancer cells without passing through these compartments, the vascular and interstitial physiology of tumors has received considerable attention in recent years. Three physiological factors responsible for the poor localization of macromolecules in tumors have been identified: (i) heterogeneous blood supply, (ii) elevated interstitial pressure, and (iii) large transport distances in the interstitium. The first factor limits the delivery of blood-borne agents to well-perfused regions of a tumor; the second factor reduces extravasation of fluid and macromolecules in the high interstitial pressure regions and also leads to an experimentally verifiable, radially outward convection in the tumor periphery which opposes the inward diffusion; and the third factor increases the time required for slowly moving macromolecules to reach distal regions of a tumor. Binding of the molecule to an antigen further lowers the effective diffusion rate by reducing the amount of mobile molecule. Although the effector cells are capable of active migration, peculiarities of the tumor vasculature and interstitium may be also responsible for poor delivery of lymphokine activated killer cells and tumor infiltrating lymphocytes in solid tumors. Due to micro- and macroscopic heterogeneities in tumors, the relative magnitude of each of these physiological barriers would vary from one location to another and from one day to the next in the same tumor, and from one tumor to another. If the genetically engineered macromolecules and effector cells, as well as low molecular weight cytotoxic agents, are to fulfill their clinical promise, strategies must be developed to overcome or exploit these barriers. Some of these strategies are discussed, and situations wherein these barriers may not be a problem are outlined. Finally, some therapies where the tumor vasculature or the interstitium may be a target are pointed out.

Localization of fluorescent Photofrin II and aluminum phthalocyanine tetrasulfonate in transplanted human malignant tumor LOX and normal tissues of nude mice using highly light-sensitive video intensification microscopy

A comparative kinetic observation of the in vivo biolocalization of Photofrin II (P-II) and aluminum phthalocyanine tetrasulfonate (AIPCS4) in a transplanted human malignant tumor LOX and in normal tissues of nude mice has been made by means of highly light-sensitive video intensification microscopy at various intervals after i.p. administration. In the human tumor LOX, transplanted to athymic nude mice, fluorescence of P-II was observed on the membrane and in the cytoplasm of tumor cells, and in the stroma 4-48 hr post-injection. From 72 hr post-injection almost all fluorescing P-II had disappeared from the membrane of the tumor cells while strong fluorescence was still found in the stroma. AIPCS4 fluorescence was seen mainly in tumorous stroma with none detected in the tumor cells. Almost no fluorescence was found in the tumorous stroma 24 hr after injection. In most normal tissues observed, P-II was eliminated at a much slower rate than AIPCS4, but the in vivo biolocalization of the 2 drugs was similar. They were observed primarily where collagenous proteins are normally found, i.e. basal lamina, collagenous connective tissue, and in keratinized epithelium, renal epithelium, mononuclear phagocyte system and on the membrane of muscular cells. In addition, AIPCS4 had a strong affinity for the bronchiogenic epithelium. In the skin, P-II was distributed in keratinized epithelium, hair, hair follicles and their accessory, collagenous connective tissue of dermis, whereas AIPCS4 was present only in hair and collagenous connective tissue of dermis. No fluorescence of P-II or of AIPCS4 was found in the skin epidermis, nor in the transitional epithelium of the bladder mucosa.

Transport of fluid and macromolecules in tumors. I. Role of interstitial pressure and convection

A general theoretical framework for transvascular exchange and extravascular transport of fluid and macromolecules in tumors is developed. The resulting equations are applied to the most simple case of a homogeneous, alymphatic tumor, with no extravascular binding. Numerical simulations show that in a uniformly perfused tumor the elevated interstitial pressure is a major cause for heterogeneous distribution of nonbinding macromolecules, because it (i) reduces the driving force for extravasation of fluid and macromolecules in tumors, (ii) results in nonuniform filtration of fluid and macromolecules from blood vessels, and (iii) leads to experimentally verifiable, radially outward convection which opposes the inward diffusion. The models are used to predict the interstitial pressure, interstitial fluid velocity, and concentration profiles as a function of radial position and tumor size. The model predictions agree with the following experimental data: (i) the interstitial pressure in a tumor is lowest at the periphery of the tumor and increases towards the center; (ii) the radially outward fluid velocity predicted by the fluid transport model is of the same order of magnitude as that measured in tissue-isolated tumors; and (iii) the concentration of macromolecules is higher in the periphery than in the center of tumors at short times postinjection; however, at later times the peripheral concentration is less than the concentration in the center. This work shows that in addition to the heterogeneous distribution of blood supply, hindered interstitial transport, and rapid extravascular binding of macromolecules (e.g., monoclonal antibodies), the elevated interstitial pressure plays an important role in determining the penetration of macromolecules into tumors. If the genetically engineered macromolecules are to fulfill their clinical promise, methods must be developed to overcome these physiological barriers in tumors.

Transport of molecules across tumor vasculature

The vascular-extravascular exchange of fluid and solute molecules in a tissue is determined by three transport parameters (vascular permeability, P, hydraulic conductivity, Lp, and reflection coefficient, sigma); the surface area for exchange, A; and the transluminal concentration and pressure gradients. The transport parameters and the exchange area for a given molecule are governed by the structure of the vessel wall. In general, tumor vessels have wide interendothelial junctions; large number of fenestrae and transendothelial channels formed by vesicles; and discontinuous or absent basement membrane. While these factors favor movement of molecules across tumor vessels, high interstitial pressure and low microvascular pressure may retard extravasation of molecules and cells, especially in large tumors. These characteristics of the transvascular transport have significant implications in tumor growth, metastasis, detection and treatment.

Extravascular transport in normal and tumor tissues

The transport characteristics of the normal and tumor tissue extravascular space provide the basis for the determination of the optimal dosage and schedule regimes of various pharmacological agents in detection and treatment of cancer. In order for the drug to reach the cellular space where most therapeutic action takes place, several transport steps must first occur: (1) tissue perfusion; (2) permeation across the capillary wall; (3) transport through interstitial space; and (4) transport across the cell membrane. Any of these steps including intracellular events such as metabolism can be the rate-limiting step to uptake of the drug, and these rate-limiting steps may be different in normal and tumor tissues. This review examines these transport limitations, first from an experimental point of view and then from a modeling point of view. Various types of experimental tumor models which have been used in animals to represent human tumors are discussed. Then, mathematical models of extravascular transport are discussed from the prespective of two approaches: compartmental and distributed. Compartmental models lump one or more sections of a tissue or body into a "compartment" to describe the time course of disposition of a substance. These models contain "effective" parameters which represent the entire compartment. Distributed models consider the structural and morphological aspects of the tissue to determine the transport properties of that tissue. These distributed models describe both the temporal and spatial distribution of a substance in tissues. Each of these modeling techniques is described in detail with applications for cancer detection and treatment in mind.

Clinical pharmacokinetics of cerebrospinal fluid

The distribution of drugs into the cerebrospinal fluid has long been considered a challenging field of investigation in 2 major respects: (a) understanding how the physicochemical properties (molecular weight, pKa, plasma protein binding) of various molecules influence their movements across such a specific structure as the blood-brain barrier; and (b) defining the relationship between cerebrospinal fluid concentrations of various drugs and their central (side) effects. An attempt has been made to review the very dispersed information presently available to offer a clinically orientated picture of this area of pharmacokinetics. Drugs acting on the central nervous system (benzodiazepines, tricyclic antidepressants, anticonvulsants, opioids), antibacterial agents, cardiovascular drugs (beta-adrenoceptor blockers and digoxin), antineoplastic drugs (mainly methotrexate), and other miscellaneous agents (corticosteroids, cimetidine, methylxanthines) are reviewed. The available evidence seems to support the conclusion that only for methotrexate and antibacterial agents does knowledge of cerebrospinal fluid pharmacokinetics have direct therapeutic implications, while the mosaic of information available for other drugs does little more than provide a partially satisfactory picture.

Preliminary model for streptozocin metabolism in mice

A mathematical model for streptozocin metabolism in mice is presented. By using the available bioassay and chemical assay data for a 300-mg/kg ip injection and the principles of physiologically based pharmacokinetics, a membrane-limited transport model with first-order kinetics was found to simulate the data adequately (average error of less than 20%). Furthermore, the first-order reaction constant derived in analyzing the bioassay data (0.009 min-1) was in close agreement with the half-life of streptozocin (1 hr) reported previously.