The antioxidant, MnTE-2-PyP, prevents side-effects incurred by prostate cancer irradiation

Abstract

Prostate cancer is the most commonly diagnosed cancer, with an estimated 240,000 new cases reported annually in the United States. Due to early detection and advances in therapies, more than 90% of patients will survive 10 years post diagnosis and treatment. Radiation is a treatment option often used to treat localized disease; however, while radiation is very effective at killing tumor cells, normal tissues are damaged as well. Potential side-effects due to prostate cancer-related radiation therapy include bowel inflammation, erectile dysfunction, urethral stricture, rectal bleeding and incontinence. Currently, radiation therapy for prostate cancer does not include the administration of therapeutic agents to reduce these side effects and protect normal tissues from radiation-induced damage. In the current study, we show that the small molecular weight antioxidant, MnTE-2-PyP, protects normal tissues from radiation-induced damage in the lower abdomen in rats. Specifically, MnTE-2-PyP protected skin, prostate, and testes from radiation-induced damage. MnTE-2-PyP also protected from erectile dysfunction, a persistent problem regardless of the type of radiation techniques used because the penile neurovascular bundles lay in the peripheral zones of the prostate, where most prostate cancers reside. Based on previous studies showing that MnTE-2-PyP, in combination with radiation, further reduces subcutaneous tumor growth, we believe that MnTE-2-PyP represents an excellent radioprotectant in combination radiotherapy for cancer in general and specifically for prostate cancer.

Figure 1. Diagram of Experimental Design.

Rats received 5 consecutive radiation doses of 7.5 Gy to the lower abdomen. Rats were injected i.p. with MnTE-2-PyP or PBS as a control throughout the study as indicated. Animals were harvested 12 weeks post-irradiation. There were four animals per group and the experiment was repeated once.

Figure 2. Pharmacokinetics of MnTE-2-PyP in the rat urogenital system.

Rats received injections of MnTE-2-PyP as outlined in Figure 1 and harvested 1 week, 2 weeks, 12 weeks and 12 weeks+1 day after the start of injections. Rats harvested at 1 and 2 weeks had not received an injection for 3 days (3 day trough). Rats harvested at 12 weeks, had not received an injection for 7 days (representing the lowest levels during the week) and 12 weeks+1 day represent rats 1 day after an injection (representing the highest levels during the week). A. Liver. B. Bowel. C. Prostate. D. Penile tissues. E. Bladder. MnTE-2-PyP was detected in the liver, bowel, prostate, penile tissue and bladder. Data represents the mean ± standard error of the mean, n = 3 per time point.

Figure 3. Irradiated rats receiving MnTE-2-PyP lost significantly less weight than irradiated rats injected with PBS.

Weights of irradiated animals throughout the course of the experiment as compared to their respective non-irradiated groups. n = 8 rats per group, asterisk (*) denotes p<0.05.

Figure 4. MnTE-2-PyP protected from irradiation-induced skin damage.

A. Representative pictures of lower abdomens at 6 weeks post-irradiation. Irradiation caused marked epilation, which MnTE-2-PyP protected. B. Irradiation caused erythma and moist desquamation in the exposed skin exposed. The skin of irradiated rats injected with MnTE-2-PyP, had significantly less severe erythma and no moist desquamation. These skin changes persisted through the course of the experiment, n = 8 rats per group, asterisk (*) denotes p<0.05.

Figure 5. Representative pictures of rat testes at 12 weeks post-irradiation.

Radiation caused the rat testes to shrink in size (∼60%, PBS RAD) as compared to non-irradiated rat testes (Control PBS). However, testes from animals treated with MnTE-2-PyP (MnTE-2-PyP RAD) were indistinguishable from non-irradiated testes.

Figure 6. MnTE-2-PyP protects prostatic and penile tissues from irradiation-induced damage 12 weeks post-irradiation.

A. Representative images of hematoxylin/eosin staining of prostate tissue (scale bar represent 5 µm). B Representative images of Masson's trichrome staining in penile tissues (scale bare represents 10 µm). The arrows indicate strong collagen staining (blue) and brown staining represents muscle fibers.

Figure 7. Measuring erectile function in rats.

Intracavernous pressure (ICP) was obtained after cavernous nerve stimulation as a measurement of erectile function 12 weeks post-irradiation. A. Representative pressure curves obtained after nerve stimulation. B. Maximum ICP obtained after cavernous nerve stimulation. Irradiation caused a significant decrease in ICP (RAD group) as compared to the non-irradiated group (PBS). MnTE-2-PyP significantly protected from the irradiation-induced loss in ICP (MnTE-2-PyP RAD). n = 8 rats/group, asterisk (*) denotes significant difference from PBS group, p<0.05 and the number symbol (#) denotes significant difference from RAD group, p<0.05.

Figure 8. MnTE-2-PyP reduces DNA oxidative damage induced by irradiation in prostate epithelial cells.

Irradiation is a known inducer of 8-OHdG, a marker of DNA oxidative damage. MnTE-2-PyP significantly reduced 8-OhdG in the prostate of irradiated animals 6 weeks post-irradiation. n = 4 rats/group, asterisk (*) denotes a significant difference from the RAD group.

Figure 9. A model demonstrating how MnTE-2-PyP could work as a radioprotectant.

We have shown that the potent antioxidant, MnTE-2-PyP protects normal tissues from irradiation-induced damage. We hypothesize that MnTE-2-PyP can inhibit injury of healthy bystander tissues by inhibiting inflammation driven by oxidative stress. We also hypothesize that MnTE-2-PyP has no effect on the killing of tumor cells due to the inability of MnTE-2-PyP to scavenge the damaging hydroxyl radical release caused by irradiation. Thus, MnTE-2-PyP can protect normal tissue from irradiation damage while not compromising the ability of irradiation to effectively kill tumor cells.