Necrosis avidity: a newly discovered feature of hypericin and its preclinical applications in necrosis imaging

Abstract

Hypericin has been widely studied as a potent photosensitizer for photodynamic therapy in both preclinical and clinical settings. Recently, hypericin has also been discovered to have a specific avidity for necrotic tissue. This affinity is also observed in a series of radiolabeled derivatives of hypericin, including [(123)I]iodohypericin, [(124)I]iodohypericin, and [(131)I]iodohypericin. Hypericin, along with other necrosis-avid contrast agents, has been investigated for use in noninvasively targeting necrotic tissues in numerous disorders. Potential clinical applications of hypericin include the identification of acute myocardial infarction, evaluation of tissue viability, assessment of therapeutic responses to treatments, and interventional procedures for solid tumors. The mechanisms of necrosis avidity in hypericin remain to be fully elucidated, although several hypotheses have been suggested. In particular, it has been proposed that the necrosis avidity of hypericin is compound specific; for instance, cholesterol, phosphatidylserine, or phosphatidylethanolamine components in the phospholipid bilayer of cellular membranes may be the major targets for its observed selectivity. Further investigations are needed to identify the specific binding moiety that is responsible for the necrosis avidity of hypericin.

Keywords: avidity; hypericin; necrosis.

Fig 1

(A) Photograph of St. John's wort. (B) Chemical structure of hypericin.

Fig 2

T1-weighted contrast-enhanced MR images were acquired 24 hours after i.v. injection of the nonporphyrin necrosis-avid contrast agent bis-Gd-DTPA-bis-indole derivative (ECIV-7) at 0.05 mmol/kg in rats with liver implantation of rhabdomyosarcoma (arrow) treated with RFA. “S” denotes the adjacent stomach. (A & A') The cross-sectional MR image and corresponding body slice in a rat with complete liver tumor ablation, clearly demonstrating an “O” type persistent contrast enhancement. (B & B') The cross-sectional MR image and corresponding body slice in a rat with incomplete liver tumor ablation, clearly demonstrating a “C” type persistent contrast enhancement. Figure modified from Ni Y et al. Eur Radiol 2006 16: 1031-1040 with permission.

Fig 3

Necrotic accumulation of ¹²³I-hypericin in a rat liver lobe with focal reperfused infarction. (A) Triphenyltetrazolium chloride (TTC) staining of a liver lobe with focal reperfused infarction. The normal liver tissue (red) and necrotic tissue (pale) were sampled for radioactivity counting. (B-D) Analyses of the 50-μm frozen liver slice adjacent to the slice used for TTC in A. (B) Autoradiography results showing that the radioactivity of ¹²³I was high in necrotic tissue, and low in normal liver tissue. (C) Histological results with H&E staining. (D) H&E staining results with enhanced contrast and brightness. The necrotic area seen upon H&E staining in C and D was closely matched with the area with high radioactivity upon autoradiography in B. Figure modified from Ni Y et al. Eur J Nucl Med Mol 2006; 33: 595-601 with permission.

Fig 4

Necrotic accumulation of ¹²³I-hypericin on the ex vivo images of a rabbit heart with infarction. (A) A TTC-stained 5-mm-thick slice with right ventricular wall attached (arrowhead), showing the transmural infarction (pale) involving the entire anterior and lateral wall of the left ventricle (thick arrow), including the posterior papillary muscle (thin arrow). (B) A TTC-stained 50-μm frozen slice with the right ventricular wall removed, showing the non-infarcted myocardium (arrowheads) at the posterior wall and the interventricular septum (superficially stained brick red). (C) A 50-μm slice subjected to autoradiography, showing a “doughnut” pattern of radioactivity uptake that appears only in the infarcted region. The highest activity (in red) is found mainly subendocardially and near the lateral border zone, where the tracer can diffuse from the blood circulation into this occlusive myocardial infarct (arrows). (D) The overlapped the image of TTC staining (B) with the autoradiographic image (C) showed a perfect match of the high radioactivity area with the infarcted region (arrows). Figure reprinted from Ni Y et al. Eur J Nucl Med Mol 2006; 33: 595-601 with permission.

Fig 5

Representative results from a hypericin-mediated dual-targeting anticancer approach in four groups of rats. (A1-A4) Group A included control animals. Implanted liver rhabdomyosarcoma tumors (R1) grew rapidly during the entire experiment. (B1-B4) Group B was treated with the vascular targeting agent CA4P. B2 shows obvious vascular shutdown and necrosis seen as rim enhancement at three days after treatment, and B3 showed rapid tumor enlargement. (C1-C4) Group C was treated with ¹³¹I-hypericin (¹³¹I-Hyp), and tumor growth was only moderately inhibited. (D1-D7) Group D was treated with a combination of CA4P and ¹³¹I-Hyp. The CA4P-induced necrosis was used as an initial target to lead ¹³¹I-Hyp to deliver radiation devastation to the second target, the adjacent viable tumor cells. D2 and D3 show that the tumor virtually stopped growing, with a persistently unenhanced tumor center. D5 shows the planar gamma scintigraph evidence of radioactivity accumulation in the necrotic tumor, which was verified by colocalized histologic slide show in D6 and autoradiograph in D7. In vivo MRI findings were verified with histopathological analysis of H&E stained slides in terms of tumor dimensions and viability. H&E staining of tumor tissue appears paler on D4 and D6 than that of surrounding healthy liver (L). In contrast, the H&E staining of healthy liver in groups A, B, and C is paler than that of tumor tissue in A4, B4, and C4, suggesting viability at risk (reduced nucleus substance) due to constant regional ionizing radiation in group D. CE-T1W-MRI = contrast-enhanced T1-weighted magnetic resonance imaging; Hyp = hypericin; i.v. = intravenous therapy; N = necrotic tumor tissue. Figure modified from Li J et al. Radiology 2011; 260(3): 799-807 with permission.