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Keywords:

  • angiogenesis;
  • vascular endothelial growth factor;
  • chemotherapy;
  • interstitial fluid pressure;
  • endothelial cell;
  • permeability

Abstract

  1. Top of page
  2. Abstract
  3. Tumor Angiogenesis and Vascular Endothelial Growth Factor
  4. Abnormalities in the Tumor Vasculature and Microenvironment
  5. Anti-VEGF Therapy
  6. Anti-VEGF Therapy and its Effects on the Abnormal Tumor Microenvironment
  7. Anti-VEGF Therapy, Reductions in IFP, and Drug Delivery in Experimental Models
  8. Conclusions
  9. Note Added in Proof
  10. REFERENCES

It is believed that impairments in delivery of antineoplastic agents to solid tumors result from abnormalities of the tumor microenvironment. Vascular endothelial growth factor (VEGF), the prototypical angiogenic molecule, is one of the main factors responsible for the development and maintenance of the aberrant tumor vascular network, which is characterized by chaotic, leaky blood vessels with high interstitial fluid pressure and inefficient blood flow. The authors proposed that anti-VEGF therapy would reduce the elevated interstitial fluid pressure in tumors, thereby improving blood flow and potentially improving delivery of cytotoxic agents to tumor cells. For the current report, the authors reviewed characteristics of the abnormal tumor vasculature created under the influence of VEGF, the resulting tumor microenvironment, how the tumor microenvironment may impede delivery of antineoplastic agents, and how the combination of anti-VEGF and cytotoxic therapy may maximize the efficacy of antineoplastic treatment regimens. Cancer 2005. © 2005 American Cancer Society.

The resistance of many solid tumors to antineoplastic agents has multifactorial causes, including both intracellular mechanisms (e.g., multidrug-resistant genes, drug export pumps, alterations in metabolic pathways, gene mutations) and factors within the tumor microenvironment.1, 2 Even tumor cells that are sensitive to chemotherapy agents in vitro often have been identified as resistant or have become resistant to chemotherapy in vivo. This disconnect between in vitro and in vivo results supports the theory that the tumor microenvironment can affect the delivery of chemotherapeutic agents to tumor cells.

The interstitial fluid pressure (IFP) within the tumor is one factor that affects drug delivery within tumors.3 Normally, hydrostatic and oncotic pressure gradients (and the resulting movement of fluids and molecules between the intravascular space and interstitium) regulate the IFP within tissues.4, 5 Blood flow and vessel permeability, along with lymphatic efferent flow, contribute to the maintenance of the hydrostatic and oncotic pressure gradients between the vasculature and the interstitium. The IFP in normal tissues typically is < 2 mm Hg and actually can be < 0 mm Hg in some tissues. Vascular abnormalities in tumors disrupt the normal homeostatic mechanisms that normally maintain IFP, leading to significant elevations of IFP; IFPs in various carcinomas have been found to be between 14 mm Hg and 30 mm Hg.6–8 The resultant equalization of hydrostatic pressures in the vascular and tumor interstitial spaces9, 10 hinders the delivery of chemotherapeutic agents to their targets; the tumor cells, as blood-borne molecules, remain in the bloodstream due to the loss of normal pressure gradient.

Tumor Angiogenesis and Vascular Endothelial Growth Factor

  1. Top of page
  2. Abstract
  3. Tumor Angiogenesis and Vascular Endothelial Growth Factor
  4. Abnormalities in the Tumor Vasculature and Microenvironment
  5. Anti-VEGF Therapy
  6. Anti-VEGF Therapy and its Effects on the Abnormal Tumor Microenvironment
  7. Anti-VEGF Therapy, Reductions in IFP, and Drug Delivery in Experimental Models
  8. Conclusions
  9. Note Added in Proof
  10. REFERENCES

The dependence of tumor growth on angiogenesis (neovascularization) has become a basic tenet of cancer biology.11 Vascular endothelial growth factor (VEGF), the prototypical proangiogenic molecule, has been implicated in various steps throughout the angiogenic process. Its initial name, vascular permeability factor, reflects its ability to increase vascular permeability; VEGF is 50,000 times more potent than histamine.12 Several variants of VEGF and its receptors have been discovered. Currently, known VEGF family members are 6 growth factors (VEGF-A, VEGF-B, VEGF-C, VEGF-D, VEGF-E, and placental growth factor) and 3 receptors (VEGFRs) (VEGFR-1 [Flt-1], VEGFR-2 [KDR/Flk-1], and VEGFR-3 [Flt-4]) (Fig. 1). VEGF-A (referred to herein as VEGF), which is the best characterized of the VEGF family members, is composed of at least 6 isoforms resulting from alternate splicing of the gene: VEGF121, VEGF145, VEGF165, VEGF183, VEGF189, and VEGF206. VEGF121 and VEGF165 are expressed commonly by solid tumors. All three VEGF receptors are transmembrane tyrosine kinase receptors and are expressed predominantly on endothelial cells. Activation of VEGFR-2 by VEGF-A, VEGF-C, or VEGF-D leads to enhanced permeability of the vasculature and to increased migration and proliferation of endothelial cells. Accordingly, VEGFR-2 is a major target for antiangiogenic therapy.13–17

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Figure 1. Vascular endothelial growth factor (VEGF) and VEGF family members. Reprinted with permission from the American Society of Clinical Oncology (Dvorak HF. Vascular permeability factor/vascular endothelial growth factor: a critical cytokine in tumor angiogenesis and a potential target for diagnosis and therapy. J Clin Oncol. 2002;20:4368–438013).

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Abnormalities in the Tumor Vasculature and Microenvironment

  1. Top of page
  2. Abstract
  3. Tumor Angiogenesis and Vascular Endothelial Growth Factor
  4. Abnormalities in the Tumor Vasculature and Microenvironment
  5. Anti-VEGF Therapy
  6. Anti-VEGF Therapy and its Effects on the Abnormal Tumor Microenvironment
  7. Anti-VEGF Therapy, Reductions in IFP, and Drug Delivery in Experimental Models
  8. Conclusions
  9. Note Added in Proof
  10. REFERENCES

Tumor blood vessels have markedly different morphology than the normal vasculature.18 During tumor angiogenesis, stimulation by angiogenic factors (predominantly VEGF) causes excess recruitment of endothelial cells and abnormalities of perivascular cells. Analysis of tumor vessels shows that they are tortuous, dilated, and saccular with disproportionate branching.18, 19 These abnormal blood vessels lead to impaired blood flow within the tumor, with areas of low flow, turbulent flow, and stasis (Fig. 2). Extrinsic compression by tumor cells also can hinder blood flow within a tumor.18 Pericyte coverage of these vessels also is abnormal, with uneven spacing and decreased attachment along the endothelium-lined vessels within the tumor.20 The heterogeneity of the aberrant vascular network leads to relatively poor oxygen delivery, with resultant regions of hypoxia and acidosis. This feature also contributes to decreased drug uptake by tumor cells and partially may explain the resistance of tumors to radiation therapy, because radiation relies on the presence of oxygen within the targeted tissue to generate oxygen free radicals.

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Figure 2. Lack of tumor enhancement with intravenous contrast during computed tomography scans of (A) colorectal liver metastases and (B) locally advanced pancreatic carcinoma demonstrate the inefficiency of the vascular networks despite the increase in vessel density found by immunohistochemical analysis.

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In addition to these physical features, VEGF has been implicated in the majority of functional changes in the tumor microenvironment that would diminish the efficiency of drug delivery to tumor cells.18, 21–24 Specifically, the vasculature formed within solid tumors is highly permeable,25 with erratic and abnormal blood flow leading to regions of increased IFP, hypoxia, or both within the tumor.19

Several hypotheses have been put forward from experimental evidence regarding the mechanism by which VEGF-A induces blood vessel hyperpermeability. Dvorak et al. proposed that macromolecules can cross the endothelium by means of vesicovacuolar organelles in a transendothelial cell pathway13, 25 and showed that these organelles are induced by VEGF-A stimulation. Others have proposed that VEGF-A opens gap junctions between endothelial cells13, 14, 26 or induces fenestrations in endothelial cells in a transcellular method of molecular transport.27–30 Nitric oxide production23 and prostaglandin I229 also have been implicated in the VEGF-mediated increase in vascular permeability. Undoubted, VEGF increases permeability through more than one mechanism. Regardless of the controversy regarding the nature of those mechanisms, it seems clear that VEGF-A results in the formation of leaky blood vessels, which would lead to extravasation of fluid and molecules into the interstitial space and a resultant increase in tumor IFP (Fig. 3). The effect of VEGF on blood vessel permeability likely has a major role in impairing drug delivery to tumors through this increase in tumor IFP. During the loss of normal oncotic and hydrostatic pressure gradients in the leaky vessels, the resultant interstitial hypertension becomes a barrier to movement of therapeutic agents into the interstitial space. This barrier would prevent antineoplastic agents from reaching their targets—the tumor cells.

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Figure 3. Schematic of the hypothetical effects of increased interstitial tumor pressure (IFP) on tumor blood flow. Tumor IFP is elevated from the development of abnormal, highly permeable tumor blood vessels under the influence of vascular endothelial growth factor (VEGF). Lymphatics are sparse and nonfunctional within tumors but may function at the tumor periphery. Administration of an anti-VEGF agent leads to reduction in IFP, less compression of the intratumoral vessels, and improved efficiency of the microvascular network, allowing more efficacious delivery of chemotherapeutic agents to the tumor cells.

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Abnormalities in the lymphatic system of tumors also contribute to increased IFP in the tumor microenvironment.31, 32 Lymphatics tend to be distended at the tumor periphery and scarce at the tumor center. The resulting impairment in lymphatic drainage would contribute to an increase in IFP as fluid efflux from the interstitial space decreases. Furthermore, increases in VEGF expression have been noted in parallel with tumor growth and increases in cell density,33 contributing to a hypothetical “vicious circle.” Thus, interstitial hypertension resulting from multiple factors within the tumor microenvironment (increased permeability, loss of oncotic and hydrostatic pressure gradients, decreases in lymphatic vessels) all could contribute to impaired drug delivery. Physically, an increase in IFP leads to collapse or narrowing of vessels within the tumor, reducing flow and impeding drug delivery. Blood tends to flow in the direction in which the least resistance is encountered, and this occurs within the tumor itself, where the IFP gradient increases with distance from the interface between tumor and normal tissue.

Notably, VEGF is not the only growth factor that affects IFP and drug delivery. The platelet-derived growth factor (PDGF) family and its receptors (PDGFRs) also have been found to regulate IFP within the tumor environment.34 Inhibition of PDGF activity leads to decreased tumor IFP and increased delivery of soluble markers and chemotherapeutic agents.34, 35 PDGF-BB and the receptor PDGFR-β have been shown to contribute to increased IFP in both normal tissue and tumor tissue.36–39 The relative contributions of VEGF and PDGF to IFP, however, remain to be determined.3,35

Anti-VEGF Therapy

  1. Top of page
  2. Abstract
  3. Tumor Angiogenesis and Vascular Endothelial Growth Factor
  4. Abnormalities in the Tumor Vasculature and Microenvironment
  5. Anti-VEGF Therapy
  6. Anti-VEGF Therapy and its Effects on the Abnormal Tumor Microenvironment
  7. Anti-VEGF Therapy, Reductions in IFP, and Drug Delivery in Experimental Models
  8. Conclusions
  9. Note Added in Proof
  10. REFERENCES

The recognition that the VEGF pathway is a key regulator of angiogenesis has led to considerable interest in exploiting its potential for therapy in oncology, and most of the antiangiogenesis treatment strategies currently in preclinical and clinical development focus on inhibition of the VEGF pathway. Several anti-VEGF strategies have been developed, including neutralizing antibodies to VEGF-A or VEGFR-2, VEGF-TRAP (a soluble VEGFR-1/VEGFR-2 hybrid), and tyrosine kinase inhibitors to the VEGFRs.40–43 Several of these agents are being tested currently in clinical trials. To our knowledge, the anti-VEGF antibody bevacizumab is the first of these agents to be approved by the U.S. Food and Drug Administration for use in combination with chemotherapy for patients with metastatic colorectal carcinoma.

One of the earliest strategies used to inhibit VEGF activity involved the use of neutralizing antibodies to VEGF-A. In preclinical studies, a murine anti-VEGF monoclonal antibody inhibited the angiogenesis and growth of human tumor xenografts.44, 45 Preclinical studies of anti-VEGFR-2 antibodies have demonstrated decreases in VEGF-induced signaling, angiogenesis, and primary and metastatic growth in a variety of different tumor systems.41, 43, 46–49 Small molecule inhibitors of tyrosine kinase activity represent another major approach to blocking VEGF-mediated angiogenesis. Several tyrosine kinase inhibitors have been developed that primarily inhibit VEGFR-2 but also are active against other VEGFRs and other tyrosine kinase receptors, including basic fibroblast growth factor receptor, epidermal growth factor receptor family members, PDGFRs, cKit, and Flt3.

Anti-VEGF Therapy and its Effects on the Abnormal Tumor Microenvironment

  1. Top of page
  2. Abstract
  3. Tumor Angiogenesis and Vascular Endothelial Growth Factor
  4. Abnormalities in the Tumor Vasculature and Microenvironment
  5. Anti-VEGF Therapy
  6. Anti-VEGF Therapy and its Effects on the Abnormal Tumor Microenvironment
  7. Anti-VEGF Therapy, Reductions in IFP, and Drug Delivery in Experimental Models
  8. Conclusions
  9. Note Added in Proof
  10. REFERENCES

Current knowledge of the effect of VEGF on vessel permeability has led to the hypothesis that anti-VEGF therapy reduces IFP in tumors, thereby improving the delivery of drugs to tumor cells. Several studies have established that combining anti-VEGF therapy with a chemotherapeutic agent is more effective compared with either agent given alone.50–55 Some investigators believe that this increase in effectiveness results from each agent having a separate effect on vascular endothelial cells56, 57; however, this concept was based on continuous, low-dose chemotherapy in combination with anti-VEGF therapy. However, combination therapies also have been effective when the chemotherapy is administered in standard regimens rather than on a continuous, low-dose schedule.

Another hypothesis that has been proposed to explain the effects of anti-VEGF therapy on the tumor vasculature and microenvironment is vascular “normalization.” As proposed by Jain,58 this hypothesis states that aberrant vessels are destroyed more easily by antiangiogenic therapy, leaving more phenotypically and functionally normal blood vessels intact within solid tumors. These remaining blood vessels are able to maintain a microenvironment better that resembles normal tissue more closely than tumor, which also may facilitate drug delivery to the tumor cells. Furthermore, if VEGF causes vessel tortuosity and dilation, then, theoretically, anti-VEGF therapy can reverse this phenotype, which also would improve the efficiency of blood flow within the tumor.

Given the weight of the evidence implicating VEGF-A in the creation of the abnormal vasculature within tumors, the interest in how anti-VEGF therapy may modify this condition is understandable. It has been shown that anti-VEGF therapy decreases IFP, reestablishes hydrostatic and oncotic pressure gradients, and, in some tumors, increases oxygen tension.24, 59 Studies of tumor vasculature after treatment with direct blockade of VEGF-A or its receptor, VEGFR-2, have shown decreases in vessel density, diameter, and permeability.24, 58–61 In one study, intravital videomicroscopy in a model of human glioblastoma directly demonstrated that SU5416, a VEGFR-2 tyrosine kinase inhibitor, led to a decrease in mean vessel density and an increase in blood flow in the remaining vessels.62 A more recent study, in which functional computed tomography was used to quantify tumor perfusion after administration of SU5416, showed significant increases in blood flow and blood volume in treated mice compared with controls.63 The authors of that study also reported a decrease in the number of newly divided endothelial cells and immature vessels in the treatment group. These findings, in combination with other evidence that anti-VEGF therapy led to regression of abnormal-appearing tumor vessels that lacked normal pericyte coverage,64 lend support to the hypothesis that anti-VEGF therapy could help return the tumor microenvironment caused by abnormal vasculature to a state more similar to the physiologic norm. Thus, anti-VEGF therapy may improve the effectiveness of drug delivery (or radiation therapy, as discussed below) by making the tumor microenvironment more favorable to the antitumor effects of these modalities.

Many of the hypotheses regarding the effects of anti-VEGF therapy on the tumor microenvironment are counterintuitive to conventional thinking regarding antiangiogenic therapy. First is the assumption that antiangiogenic therapy is intended to restrict blood supply to the tumor and, hence, to kill tumor cells, an idea that contradicts the hypothesis that anti-VEGF therapy actually restores hydrostatic and oncotic pressure gradients, thus, leading to decreased IFP and improved blood flow to the tumor. This effect may be more pronounced with the use of small-molecule therapeutic agents (chemotherapy or tyrosine kinase inhibitors as opposed to antibodies), because delivery of small molecules to the tumor cells may be more dependent on pressure gradients. Improved flow also may facilitate oxygen delivery to tumors. According to this line of thinking, the overall result of anti-VEGF therapy would be improved drug delivery and possibly sensitization of tumors to radiation therapy.

Two other hypotheses regarding the effects of anti-VEGF therapy on the tumor microenvironment also may be considered counterintuitive to conventional thinking. If anti-VEGF therapy leads to improved flow and decreased IFP, then the question arises whether the resulting improvement in oxygenation and nutrient delivery actually may be conducive to further tumor growth. Although such a result is possible in theory, no study conducted to date has noted any promotion of tumor growth with anti-VEGF therapy, probably because the anti-VEGF therapy simultaneously is “normalizing” the vascular network, inhibiting new vessel growth, and halting tumor expansion. Finally, it is possible that chronic anti-VEGF therapy eventually may decrease permeability to chemotherapeutic drugs if only vessels with pericyte coverage (i.e., those resistant to VEGF therapy) remain. However, pericytes in tumor vasculature are associated more loosely with endothelial cells and do not adhere as tightly as they do on normal vessels.20 This abnormal pericyte coverage of tumor vasculature makes it unlikely that anti-VEGF therapy possibly could hinder drug delivery by decreasing permeability.

Another issue to be addressed is the question of whether permeability should be increased to improve the delivery of a drug within a tumor if a decrease in permeability limits drug perfusion. We hypothesize that, once a VEGF-driven tumor reaches a certain size, the vasculature already is highly permeable, but the point at which that happens is unknown. Furthermore, once permeability has increased to the point where the pressure in the tumor equals that in the microvasculature, further increases in permeability (and, thus, in drug delivery to the tumor cells) are not possible. However, at this late stage, the changes associated with a decrease in VEGF expression (decreased IFP and “normalization” of the vasculature) may restore the microvasculature pressure gradients and, hence, facilitate uptake of small molecules by the tumor.

We want to make a distinction between anti-VEGF therapy and the broader category of antiangiogenic therapy, which is more poorly defined at this time. Although anti-VEGF therapy may have antiangiogenic actions, as we have discussed here, it has other effects as well. Antiangiogenic therapy is intended to target endothelial cells and may inhibit blood vessel formation and, at times, even may lead to the induction of endothelial cell apoptosis. Thus, although anti-VEGF therapy may be antiangiogenic in a classic sense, it also has other effects on the functionality of the vascular bed.

Anti-VEGF Therapy, Reductions in IFP, and Drug Delivery in Experimental Models

  1. Top of page
  2. Abstract
  3. Tumor Angiogenesis and Vascular Endothelial Growth Factor
  4. Abnormalities in the Tumor Vasculature and Microenvironment
  5. Anti-VEGF Therapy
  6. Anti-VEGF Therapy and its Effects on the Abnormal Tumor Microenvironment
  7. Anti-VEGF Therapy, Reductions in IFP, and Drug Delivery in Experimental Models
  8. Conclusions
  9. Note Added in Proof
  10. REFERENCES

The limited efficacy of anti-VEGF monotherapy for most solid malignancies (except for renal cell carcinoma) in clinical trials has stimulated interest in combining antiangiogenic therapy with conventional antineoplastic therapy.65 The rationale for such combinations was to target two compartments of the tumor—the endothelium and the tumor cells.66 Later studies by Kerbel and colleagues suggested that chronic, low-dose (“metronomic”) chemotherapy may have direct effects on the vasculature and that anti-VEGF therapy may augment these effects.56, 57 Growing evidence that anti-VEGF therapy may decrease IFP or normalize the tumor vasculature, thereby improving the function of the tumor vascular bed, has led investigators in several preclinical trials to explore whether anti-VEGF therapy indeed does induce these changes in tumors.

Multiple studies in both in vitro and in vivo models have established that combining anti-VEGF therapy with a chemotherapeutic agent is more successful in terms of reducing tumor volume, decreasing rates of metastasis, and increasing survival compared with monotherapy using individual agents alone.50–55 VEGFR-2 antagonists given in combination with various chemotherapeutic agents have led to significant reductions in vessel density and neovascularity in several carcinoma models.46, 67–69 One group of investigators,70 using a murine intracerebral tumor model, measured interstitial fluid concentrations of a single dose of the alkylating agent temozolomide given after a 9-day treatment with SU5416. The SU5416 treatment had no effect on tumor size but was associated with much higher temozolomide concentrations in the intracerebral model, but not in a subcutaneous model, demonstrating the importance of orthotopic implantation in the use of preclinical animal models.

More recently, the effect of pretreatment with the monoclonal anti-VEGF antibody A4.6.1 was studied in a subcutaneous model of colon carcinoma.71 End points in that study were tumor chemotherapeutic drug concentrations, tumor growth, tumor vessel density, and vascular function. Two doses of A4.6.1 were given 3 days apart, followed by a bolus dose of irinotecan 3 days after the second dose of A4.6.1. Antibody pretreatment led to a trend toward an increase in intratumoral irinotecan levels (P = 0.09) and a decrease in overall tumor vessel density. Measurements of a marker of the vascular perfusion of tumor tissue (Hoechst H33342) suggested that A4.6.1 pretreatment was associated with increased tumor perfusion, an observation that is consistent with the apparent increase in drug uptake. Previous studies72 with the same anti-VEGF antibody led the investigators to attribute the putative increase in tumor irinotecan concentrations to a decrease in IFP and to pruning of aberrant vasculature.

A smaller body of work has focused on combining anti-VEGF therapy with radiation therapy for the treatment of solid tumors. Early work with angiostatin by Gorski and colleagues indicated that VEGF protected tumors from radiation and that the protective effect could be decreased with antiangiogenic therapy.73–76 Others showed that combining radiation with the VEGFR-2 inhibitor DC101 decreased the required radiation dose but did not find any changes in tumor oxygenation (as measured by partial oxygen tension [pO2]) or in radiosensitization.77 Other investigators have shown that combination therapy both delays tumor growth and improves radiosensitization.78, 79 Anti-VEGF therapy also may reduce the radioresistance of tumors with intratumoral hypoxia by decreasing IFP and increasing pO2, although an increase in pO2 has not been seen in all types of tumors.72 Thus, VEGF-blocking therapy and subsequent improvements in oxygenation of tumor tissue due to improved flow well may enhance tumor response to radiation therapy. An additional benefit of anti-VEGF therapy with regard to radiation therapy, improved delivery of radiosensitizing agents,80 also is noteworthy.

A recent Phase I study60 integrated these ideas at the clinical level. In that study, six patients with locally advanced rectal carcinoma were given VEGF blockade with bevacizumab followed by conventional neoadjuvant chemoradiation therapy. Significant decreases in tumor IFP were noted along with decreases in tumor perfusion and mean vessel density. None of the six patients experienced any dose-limiting toxicity, and all underwent surgery without complications. The authors of that report hypothesized that the normalization of the tumor microenvironment and vasculature induced by bevacizumab resulted in retardation of the shedding of metastatic cells, sensitization of the endothelium to cytotoxic agents, and improved delivery of therapeutic agents within the tumors.

Combinations of anti-VEGF therapy with other antigrowth factor therapies that decrease IFP also show promise. In a series of studies of imatinib mesylate, which is a PDGFR tyrosine kinase inhibitor that also inhibits KIT, ABL, BCR-ABL, and ARG,81–83 Pietras and colleagues showed that inhibiting PDGFR reduced tumor IFP34, 35, 84–86 and increased tumor uptake of the freely diffusible tracer substance 51Cr-EDTA in a rat colon carcinoma model.85 In murine models of thyroid carcinoma, reductions in tumor IFP after imatinib were associated with increased tumor concentrations of paclitaxel86 and a three-fold tumor-specific increase in levels of the anticancer agent epothilone B.84 Other ways of decreasing IFP also were capable of enhancing the uptake of chemotherapy agents or molecular markers (Table 1), further substantiating the proposed mechanism of action of anti-VEGF therapy and its role in improving the effectiveness of chemotherapy.

Table 1. Drugs Known to Reduce Interstitial Fluid Pressure
DrugAnimal: ModelAgent for which delivery was improved
  1. VEGF: vascular endothelial growth factor; VEGFR-2: VEGF receptor 2; PDGFR: platelet-derived growth factor receptor; EDTA: ethylenediamine tetraacetic acid; TGF-β: transforming growth factor-β.

Anti-VEGF antibodies  
 A4.6.1 (Wilders et al., 200471)Mouse: Subcutaneous human colon carcinomaIrinotecan (P = 0.09)
 Bevacizumab (Lee et al., 200072)Mouse: Subcutaneous xenografted human colon carcinoma and gliomaRadiation (through increased pO2)
VEGFR-2 tyrosine-kinase inhibitor  
 SU5416 (Ma et al., 200370)Mouse: Intracerebral human gliomaTemozolomide
Anti-PDGFR tyrosine-kinase inhibitor  
 Imatinib mesylate (Pietras et al., 2002, 200384, 86)Rat: Subcutaneous human thyroid carcinomaPaclitaxel, epothilone B
 Imatinib mesylate (Pietras et al., 200185)Rat: Subcutaneous rat colon carcinoma51Cr-EDTA
Other  
 Dexamethasone (Kristjansen et al., 199389)Mouse: Subcutaneous human colon carcinomaNot measured
 Prostaglandin-E1 (Rubin et al., 200090; Salnikov et al., 200391)Rat: Subcutaneous rat colon carcinoma5-Fluorouracil, 51Cr-EDTA
 TGF-β receptor chimeric protein (Lammerts et al., 200292)Mouse: Subcutaneous human thyroid cancerNot measured

Finally, chemotherapy-induced reductions in IFP also may be caused by tumor cell apoptosis that leads to reduced “crowding” of tumor cells within the tumor mass. The presence of tumor cells themselves can increase IFP through mechanical pressure. Agents that specifically target tumor cells can “relieve” this pressure and decrease IFP. Paclitaxel and docetaxel led to a reduction in density of intact neoplastic cells in a murine model that involved subcutaneous implantation of HSTS-26T human sarcoma cells.87 With this reduction in tumor cell density came increases in the diameter of tumor vessels within the dorsal skin-fold chambers and reductions in IFP and microvascular pressure.9, 10, 88 It is interesting to note that increased VEGF expression has been shown in parallel with tumor cell density,33 suggesting that “effective” chemotherapy may contribute to reduced IFP through two mechanisms: directly, by inducing tumor cell death and decreasing tumor cell density, and indirectly, by inducing a decrease in VEGF levels triggered by the decrease in cell density. Thus, chemotherapy (and perhaps radiation therapy as well) may enhance the actions of anti-VEGF therapy further as tumor cells die.

Conclusions

  1. Top of page
  2. Abstract
  3. Tumor Angiogenesis and Vascular Endothelial Growth Factor
  4. Abnormalities in the Tumor Vasculature and Microenvironment
  5. Anti-VEGF Therapy
  6. Anti-VEGF Therapy and its Effects on the Abnormal Tumor Microenvironment
  7. Anti-VEGF Therapy, Reductions in IFP, and Drug Delivery in Experimental Models
  8. Conclusions
  9. Note Added in Proof
  10. REFERENCES

Efficient, specific delivery of chemotherapeutic agents to solid tumors in the presence of abnormal tumor microenvironments continues to be a problem without a satisfactory solution. VEGF has a major role in creating the abnormal vasculature within solid tumors that is associated with increased tumor IFP, impeding effective drug delivery to tumor cells. Evidence is emerging in support of the hypothesis that anti-VEGF therapy can improve blood flow and decrease IFP by restoring oncotic and hydrostatic pressure gradients. These effects establish a tumor microenvironment that is more conducive to the effective delivery and uptake of therapeutic agents. Therefore, further research into the effects of combining antiangiogenic and antineoplastic therapy holds much promise for helping to solve the difficult problem of modifying the tumor microenvironment to favor optimal drug delivery to the targeted tumor cells. 4

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Figure 4. (A) Treatment with antivascular endothelial growth factor (anti-VEGF) antibody reduced the interstitial tumor pressure (IFP) by > 70% in xenografts of U87 glioblastoma multiforme cells and LS174T colon carcinoma cells compared with tumor size-matched controls (adapted from Lee CG, Heijn M, di Tomaso E, et al. Anti-vascular endothelial growth factor treatment augments tumor radiation response under normoxic or hypoxic conditions. Cancer Res. 2000;60:5565–557072). (B) In a separate study, anti-VEGFR2 antibody also reduced IFP in U87 and 54A (lung carcinoma) xenografts (adapted from Tong RT, Boucher Y, Kozin SV, Winkler F, Hicklin DJ, Jain RK. Vascular normalization by vascular endothelial growth factor receptor 2 blockade induces a pressure gradient across the vasculature and improves drug penetration in tumors. Cancer Res. 2004;64:3731–373624).

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Note Added in Proof

  1. Top of page
  2. Abstract
  3. Tumor Angiogenesis and Vascular Endothelial Growth Factor
  4. Abnormalities in the Tumor Vasculature and Microenvironment
  5. Anti-VEGF Therapy
  6. Anti-VEGF Therapy and its Effects on the Abnormal Tumor Microenvironment
  7. Anti-VEGF Therapy, Reductions in IFP, and Drug Delivery in Experimental Models
  8. Conclusions
  9. Note Added in Proof
  10. REFERENCES

In a recently published study by Winkler et al., the authors demonstrated that there is an optimal period for the delivery of radiation therapy in combination with anti-VEGFR-2 therapy, the “normalization window,” in a model of glioblastoma.93 In that study, radiation therapy administered Days 4–6 after initiation of an anti-VEGFR-2 antibody (DC101) in mice bearing human glioblastoma xenografts led to a synergistic decrease in tumor growth delay. The combination of DC101 and radiation given outside of this normalization window showed no more than an additive effect on tumor growth delay. Moreover, the period during which radiation therapy was most effective coincided with the time of maximal tumor oxygenation. Examination of the tumor vasculature during the normalization window demonstrated increased pericyte coverage of endothelial cells through up-regulation of angiopoietin-1 and a thinning of the pathologically thick basement membrane. These changes were observed only within the normalization window. The authors hypothesized that these pericyte-fortified blood vessels are more efficient compared with the untreated tumor vasculature, thus enhancing oxygen delivery to the tumor.

We believe this recent study by Winkler and colleagues provides more insight into the biologic complexity of scheduling of antiangiogenic therapy when it is combined with other agents. In addition, a logical extension of the results of this study is that a normalization window also exists for improved delivery of chemotherapeutics in combination with VEGFR-2 blockade, an area deserving of further research.

REFERENCES

  1. Top of page
  2. Abstract
  3. Tumor Angiogenesis and Vascular Endothelial Growth Factor
  4. Abnormalities in the Tumor Vasculature and Microenvironment
  5. Anti-VEGF Therapy
  6. Anti-VEGF Therapy and its Effects on the Abnormal Tumor Microenvironment
  7. Anti-VEGF Therapy, Reductions in IFP, and Drug Delivery in Experimental Models
  8. Conclusions
  9. Note Added in Proof
  10. REFERENCES
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