The microvasculature that supplies human and mouse cancers, and for that matter healing wounds, is both similar and similarly heterogeneous (1-3). It includes at least six distinct blood vessel types (4,5). Four of these arise from preexisting venules or capillaries and are the product of angiogenesis: mother vessels, glomeruloid microvascular proliferations, capillaries, and vascular malformations. The remaining two blood vessel types, feeding arteries and draining veins, arise from preexisting arteries and veins and so result from arterio-venogenesis. Most of these vessel types are present in all human cancers thus far examined. Further, an adenovirus expressing VEGF-A164 (Ad-VEGF-A164) can replicate each of these vessel types in immunodeficient nude mice. Studies with the Ad-VEGF-A164 model have allowed us to demonstrate the temporal sequence and kinetics of new tumor blood vessel formation and to explore the mechanisms of anti-VEGF/VEGFR therapy.

Anti-VEGF/VEGFR drugs have added significantly to the cancer therapy armamentarium, but, unfortunately, are not the wonder drugs that had been hoped for. There are at least three reasons for their limited success: 1. Some tumors, e.g., some lung metastases, do not require new blood vessels for tumor survival and growth. 2. Other tumors are able to survive in extreme hypoxia and so require minimal new blood vessel support. And 3. Anti-VEGF/VEGFR does not target all tumor blood vessels equally; in fact, only a subset is affected, particularly mother vessels (6,7).

Also, the mechanisms by which anti-VEGF/VEGFR drugs act are not well defined. The generally accepted view is that they kill the endothelial cells of affected blood vessels. We now propose an alternate and/or additional explanation, namely, that vessel killing can be an indirect effect. Anti-VEGF/VEGFR drugs interfere with VEGF signaling; as a result they inhibit eNOS expression and so reduce expression of its product, NO. NO is a potent arterial dilator (8) that serves to maintain normal arterial tone. The reduced NO resulting from anti-VEGF/VEGFR therapy is expected to cause arterial contraction, and is likely responsible for the hypertension that is commonly observed in patients receiving these drugs (9). We have now found that these drugs act potently on the feeding arteries that supply downstream mother vessels. Mother vessels are lined only by endothelial cells, without smooth muscle coverage, and are therefore uniquely susceptible to damage when immediately upstream feeding arteries contract. These findings could have therapeutic significance. Treating patients receiving anti-VEGF/VEGFR drugs with anti-hypertensive therapy could interfere with their beneficial effects, which may be mediated through the selective contraction of tumor feeding arteries.

Finally, anti-angiogenesis therapy to date has almost exclusively targeted VEGF and its receptors. This focus may be too narrow. There is a need for new vascular targets apart from the VEGF/VEGFR axis, if anti-vascular therapy is to have a more important role in cancer therapy. One such target that we have been studying is the tetraspanin-like plasma membrane protein, TM4SF1 (10-12).

References:

1. Nagy JA, Chang SH, Shih SC, Dvorak AM, Dvorak HF. Heterogeneity of the tumor vasculature. Semin Thromb Hemost 2010;36(3):321-31.

2. Nagy JA, Dvorak HF. Heterogeneity of the tumor vasculature: the need for new tumor blood vessel type-specific targets. Clin Exp Metastasis 2012;29(7):657-62.

3. Dvorak HF. Tumors: Wounds That Do Not Heal-Redux. Cancer Immunol Res 2015;3(1):1-11.

4. Nagy JA, Chang SH, Dvorak AM, Dvorak HF. Why are tumour blood vessels abnormal and why is it important to know? Br J Cancer 2009;100(6):865-9.

5. Nagy JA, Dvorak AM, Dvorak HF. VEGF-A and the induction of pathological angiogenesis. Annu Rev Pathol 2007;2:251-75.

6. Sitohy B, Nagy JA, Dvorak HF. Anti-VEGF/VEGFR Therapy for Cancer: Reassessing the Target. Cancer Res 2012;72(8):1909-14.

7. Sitohy B, Nagy JA, Jaminet SC, Dvorak HF. Tumor-surrogate blood vessel subtypes exhibit differential susceptibility to anti-VEGF therapy. Cancer research 2011;71(22):7021-8.

8. Forstermann U, Sessa WC. Nitric oxide synthases: regulation and function. Eur Heart J 2012;33(7):829-37, 37a-37d.

9. Rini BI, Cohen DP, Lu DR, Chen I, Hariharan S, Gore ME, et al. Hypertension as a biomarker of efficacy in patients with metastatic renal cell carcinoma treated with sunitinib. J Natl Cancer Inst 2011;103(9):763-73.

10. Lin CI, Merley A, Sciuto TE, Li D, Dvorak AM, Melero-Martin JM, et al. TM4SF1: a new vascular therapeutic target in cancer. Angiogenesis 2014;17(4):897-907.

11. Shih SC, Zukauskas A, Li D, Liu G, Ang LH, Nagy JA, et al. The L6 protein TM4SF1 is critical for endothelial cell function and tumor angiogenesis. Cancer Res 2009;69(8):3272-7.

12. Zukauskas A, Merley A, Li D, Ang LH, Sciuto TE, Salman S, et al. TM4SF1: a tetraspanin-like protein necessary for nanopodia formation and endothelial cell migration. Angiogenesis 2011;14(3):345-54.

Citation Format: Harold F. Dvorak. Heterogeneity of the tumor vasculature: Why doesn't anti-VEGF/VEGF receptor therapy work better? [abstract]. In: Proceedings of the AACR Special Conference: Tumor Angiogenesis and Vascular Normalization: Bench to Bedside to Biomarkers; Mar 5-8, 2015; Orlando, FL. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(12 Suppl):Abstract nr IA01.