Two Endogenous Antiangiogenic Inhibitors, Endostatin and Angiostatin, Demonstrate Biphasic Curves in their Antitumor Profiles

Abstract

Angiogenesis refers to growth of blood vessels from pre-existing ones. In 1971, Folkman proposed that by choking off the blood supply to tumors, they are starved, leading to their demise. A few years ago, the monoclonal antibody Avastin became the first antiangiogenic biological approved by FDA, for treatment of cancer patients. Two other antiangiogenic endogenous protein fragments were isolated in Folkman's laboratory more than a decade ago. Here, we present a short review of data demonstrating that angiostatin and endostatin display a biphasic antitumor dose-response. This behavior is common among a large number of antiangiogenic agents and the reduced effectiveness of antiangiogenic agents at high dose rates may be due to suppression of growth of new vessels carrying the agent into the critical region around the tumor.

Keywords: Angiogenesis; Angiostatin; Biphasic; Endostatin.

FIGURE 1.

The full sequence of human plasminogen containing 5 kringle domains. The N-terminus of angiostatin resides at amino acid 78 (valine). The C-terminus of original angiostatin was not determined, however, based on its molecular size was estimated to be K1–3. The recombinant angiostatin consists of K1–4.

FIGURE 2.

Crystal structure of endostatin. The black dot represents zinc atom (B). The orange region corresponds to 25 amino acid peptide at the C-terminus of endostatin (A), which mimics the antitumor activity of the protein. This peptide contains three histidines responsible for zinc binding of endostatin.

FIGURE 3.

Pharmacokinetics of endostatin and Fc-endostatin in mice. Endostatin (100 μg) was injected s.c. into C57Bl/6J mice and concentrations of the circulating protein were monitored by ELISA (CytImmune Sciences, Rockville, MD). A) hFc-endostatin (closed circles) and human endostatin (closed squares). B) mFc-endostatin (closed circles) and mouse endostatin (closed squares). The measured concentrations of mouse endostatin were corrected for baseline endostatin (60 ng/mL).

FIGURE 4.

Biphasic anti-tumor activity of endostatin. A) Treatment of SCID mice, bearing human ASPC-1 pancreatic tumors with daily injection of clinical grade endostatin (Celik et al. 2005) (Reprinted with permission of publisher and authors). Mean (+ SD) tumor volume after a 16-day treatment with different dosages of endostatin. B_a) Similar to (A), except Fc-endostatin was employed. Note the difference of endostatin doses required to achieve antitumor effects with clinical grade endostatin and Fc-endostatin. C_a) Treatment of SCID mice bearing human melanoma A2058 tumor cells with Fc-endostatin. *P <0.001. B_b and C_b refer to tumor inhibition percentages (T/C) for the same data in Fig. B_a and C_a respectively (Lee et al. 2008). Number of mice in each group is designated by “n”.

FIGURE 5.

Biphasic anti-tumor activity of angiostatin. Mouse Fc-angiostatin was injected in C57Bl/6J mice bearing Lewis-Lung Carcinoma (LLC). This is an aggressive tumor model and a shorter period of time allows one to obtain significant tumor size differences as a result of treatment.