The treatment of solid tumor cancers is notoriously restricted by poor penetration of chemotherapeutic drugs into tumor tissues (reviewed in (1
)). A major barrier to drug delivery is the structurally and functionally abnormal tumor vasculature. They are porous, disorganized, and dysfunctional, and result in an abnormal tumor microenvironment characterized by interstitial hypertension, hypoxia, and acidosis ((2
) and reviewed in (3
)). These properties prevent efficient delivery of therapeutics to tumor cells. As such, there is considerable interest in developing adjunctive modalities to enhance the delivery of drugs into tumor tissues. Agents that target tumor endothelium have been at the forefront of this endeavor, in particular agents that target vascular endothelial growth factor (VEGF) signaling. Anti-VEGF therapies induce tumor vascular remodeling accompanied by functional changes that include decreased tumor interstitial fluid pressure and re-establishment of a hydrostatic pressure gradient across the normalized vascular wall which facilitates drug transport (4
). Looking at albumin perfusion and diffusion into tumor tissues, Tong et al reported a significant decrease in albumin permeability across the tumor vascular walls after treatment with anti-VEGFR2 antibodies, but deeper penetration of albumin into tumor tissues (4
). This treatment hypothesis has been supported by phase III clinical studies showing that the addition of anti-VEGF antibodies significantly improves the efficacy of chemotherapeutic agents in cancers of the breast, colon, and lung (5
Dynamic contrast enhanced MRI (DCE-MRI) using macromolecular contrast media with pharmacokinetic modeling has proven highly instrumental in characterizing and confirming the functional changes in tumor vasculature, in particular trans-endothelial permeability, following treatment with vascular targeting drugs. Elevated Kps is a pathologic feature of hyperpermeable tumor vasculature since it is not usually seen in benign lesions, but frequently seen in malignant tumors (8
). Studies by us and other groups using DCE-MRI in preclinical models confirm that several classes of agents that target VEGF signaling induce a reduction in tumor trans-endothelial permeability (Kps) (9
). These effects are evident as early as 24 hours of treatment in some models.
Tumor vasculature is also regulated by signaling pathways other than the VEGF receptors and these alternate pathways can potentially be targeted to enhance vascular function. The human epidermal growth factor receptor (HER) family of tyrosine kinases (TKs) are widely expressed in adult tissues and signal diverse cellular functions including proliferation, differentiation, metabolic functions and cell survival. When deregulated in certain cancers, HER kinases promote uncontrolled growth, invasiveness, hypermetabolism and anti-apoptotic attributes (15
). HER kinases also mediate some of the angiogenic properties of cancer cells (16
). Importantly, HER kinases are involved in angiogenic signaling both within tumor cells through regulation of VEGF expression, and within endothelial cells through regulation of Akt signaling (18
). This suggests that inhibitors of HER TKs may target both the tumor and microenvironment compartments and may have enhanced ability to improve tumor vascular function.
HER TKIs can enhance the anti-tumor effects of chemotherapeutic agents in preclinical models (23
). But this effect is modest in preclinical models and is not seen in phase III clinical studies (25
). Recent evidence suggests that Akt-driven feedback signaling through HER3 significantly attenuates the effects of TKIs and much higher potency or high dose TKIs are required to effectively suppress HER family signaling (27
). This intrinsic resistance is overcome by use of HER TKIs at high dosages and has significant implications for the future use of these agents. The clinical use of TKIs at high doses given daily is impractical due to their dose-limiting toxicities. However when used as chemosensitizers, they need not be used in continuous daily dosing. For this purpose they can be used in a brief one or two day dose which is more tolerable and allows higher doses to be administered. The brief but higher dose can be administered immediately prior to chemotherapy optimizing their use as chemosensitizers. The higher dose also allows better penetration of the TKI into tumor tissues and inactivation of targets within tumor cells and tumor vasculature, optimally priming the microenvironment for chemotherapeutic delivery. In this paper we report the effects of the HER TKI gefitinib on tumor vascular function by dynamic contrast enhanced MRI (DCE-MRI.). We compare the effects of a brief high dose with a lower continuous dose and find highly favorable improvements in tumor vascular function in the time period immediately following a high dose, but not with continuous low dose therapy. These effects include a transient reduction in tumor Kps accompanied by an increase in fPV and slight decrease in tumor volume immediately following the pulse high dose of gefitinib. The favorable effects of high dose gefitinib on chemodelivery are corroborated by tumor growth studies and by fluorescence microscopic analysis of tumor sections.