Dispase facilitates posterior vitreous detachment during vitrectomy in young pigs

PURPOSE

To evaluate the role of intravitreal dispase in conjunction with pars plana vitrectomy to facilitate the creation of a posterior vitreous detachment (PVD) in young pig eyes.

METHODS

Twenty-four eyes of 24 animals were randomized to receive an intravitreal injection of dispase (50 microg/0.05 mL) or phosphate buffered saline (PBS) immediately after core vitrectomy and before attempted creation of a posterior cortical vitreous detachment. Following a 15-minute waiting period, surgical creation of a posterior vitreous separation was attempted by aspiration of the posterior vitreous immediately adjacent to the optic disk. Eyes were evaluated postoperatively by clinical examination (1, 4, and 8 weeks) and electroretinography (4 and 8 weeks), after which they were enucleated for light, scanning, and transmission electron microscopy.

RESULTS

Based on intraoperative findings and postoperative scanning electron microscopy, eyes receiving intravitreal dispase exhibited a higher incidence of PVD compared to eyes receiving PBS (P = 0.029). Electroretinographic responses recorded at postoperative weeks 4 and 8 were similar in both dispase and PBS eyes compared to the unoperated fellow eyes. Clinical examinations, including indirect ophthalmoscopy, were indistinguishable between the PBS eyes and 11 of 12 eyes in the dispase group. Light and transmission electron microscopy demonstrated no differences in the retina between the dispase eyes and the PBS operated controls.

CONCLUSION

Dispase is a useful adjunct in facilitating surgical creation of a PVDin young pig eyes.

Early loss of synaptic protein PSD-95 from rod terminals of rhodopsin P347L transgenic porcine retina

Retinitis pigmentosa (RP), a type of retinal degeneration involving first rod and then slow cone photoreceptor degeneration, can be caused by any of a number of mutations in different genes. In the cases of mutations affecting rod-specific genes such as rhodopsin, it is unclear how the mutations may cause degeneration of cones. We have used the porcine retina, which is rod-dominated and has an abundance of cones, to study the mutation-induced changes in both rod and cone photoreceptors. Like patients with the same mutation, rhodopsin P347L transgenic swine manifest rod-cone degeneration. In addition, the rod bipolar cells fail to form synaptic connections with rods; instead, they form ectopic synapses with cones. The mechanisms that prevent the formation of the rod-rod bipolar cell synaptic connection are not known. We used specific antibodies and immunocytochemistry to show that the synaptic protein, PSD-95, is present in both normal and transgenic porcine retinas. During neonatal development, however, PSD-95 is lost from rod terminals in the transgenic swine. This loss is virtually complete (90%) by postnatal day 5, at a time when greater than 80% of rod cell bodies still remain. Furthermore, the remaining rods retain their outer segments and their gross morphology appears relatively normal. In contrast, PSD-95 expression continues in cone terminals, even in 10-month-old transgenic swine, where the rods have all disappeared and the cones show signs of severe degeneration. These results suggest that loss of PSD-95 may not be a general consequence of the deteriorating cell. Rather, the very early and selective loss of PSD-95 from the rod terminals may be causally related to the absence of rod-rod bipolar cell synapses in the rhodopsin P347L transgenic retina.

Evaluation of retinal toxicity and efficacy of anti-cytomegalovirus and anti-herpes simplex virus antiviral phosphorothioate oligonucleotides ISIS 2922 and ISIS 4015

Retinal toxicity of ISIS 2922 and ISIS 4015, phosphorothioate oligonucleotides complementary to human cytomegalovirus (CMV) and herpes simplex virus (HSV) RNA, were evaluated. The intravitreal concentration of ISIS 2922 found not to cause permanent toxic changes in the rabbit retina was 10 microM and in the pig retina, 5 microM. The 3 microM concentration was associated with a transient inflammatory response, and 1 microM caused no retinal toxicity or inflammation. ISIS 4015 showed very mild toxicity with no permanent retinal changes and very mild inflammation at doses of 10 microM; this dose was effective in ameliorating or preventing HSV-1 retinitis when injected 1 day and 1 week prior to virus inoculation. These oligonucleotides have a low intraocular therapeutic index. Attempts to improve the therapeutic index of these compounds are indicated. Only a clinical trial can determine the toxicity profile of ISIS 2922 for the treatment of CMV retinitis.

Evaluation of retinal toxicity and efficacy of the anticytomegalovirus compound 2-amino-7-[(1,3-dihydroxy-2-propoxy)methyl]purine

Compound 2242, also known as 2-amino-7-[(1,3-dihydroxy-2-propoxy)methyl]purine, is the first known antivirally active nucleoside analog with the side chain substituted at the N-7 position of the purine ring system. Our purpose was to evaluate its retinal toxicity and assess the efficacy of its highest nontoxic concentration in a rabbit model of herpes simplex retinitis. Concentrations of the drug from 0.5 to 2,000 microM were injected intravitreally in twelve New Zealand White rabbits. Fundoscopic, histologic, and electrophysiologic data revealed no evidence of toxicity even at the highest dose of the compound. Dutch pigmented rabbits (n = 34) had their left eyes injected with herpes simplex virus type 1 3 days after, concurrently, or 3 days before intravitreal injection of either 2,000 microM compound 2242 or 480 microM ganciclovir (final concentration in the eye). Both compound 2242 and ganciclovir were equally effective compared with saline when administered simultaneously with the virus (P < 0.0001). In the 3-day pretreatment paradigm, compound 2242 was superior to ganciclovir (P < 0.04), but there was no clear difference between the two with regard to their effects on an established infection. The pharmacokinetics of compound 2242 in 10 rabbits injected intravitreally with 30 microM showed an intravitreal half-life of 8 h. This compound, which may be orally active in its pro form, has a very high therapeutic index in the eye and is more efficient than ganciclovir in this animal model of herpes retinitis.

Antiviral effect in human cytomegalovirus-infected cells, pharmacokinetics, and intravitreal toxicology in rabbits of acyclovir diphosphate dimyristoylglycerol

Acyclovir diphosphate dimyristoylglycerol (ACVDP-DG) is a lipid prodrug which is active against ACV-resistant strains of herpes simplex virus because of its intracellular metabolism to ACV monophosphate. In human cytomegalovirus (HCMV)-infected MRC-5 cells, ACVDP-DG was ninefold more active than ACV. When liposomal [8-3H]ACVDP-DG was injected intravitreally at the maximum nontoxic dose of 1 mumol in rabbits, the drug remained above its estimated 90% HCMV-inhibitory concentration for 18 days. Intravitreal ganciclovir persists above its 90% inhibitory concentration for only 1 to 2 days. ACVDP-DG may be useful as a local treatment for HCMV retinitis.

Absence of infectious retinitis after injection of human cytomegalovirus into rabbit eyes

The rabbit model of human cytomegalovirus (HCMV) retinitis was evaluated by preretinal and intravitreal injection of HCMV into rabbit eyes. Ocular disease was evaluated by indirect ophthalmoscopy, histopathology, and immunohistochemistry. Vitritis, optic nerve congestion, multiple small white infiltrates in the cortical vitreous or on the retinal surface, and retinal detachments were seen. Histopathologic examination showed inflammatory cell infiltration in the preretinal vitreous, optic nerve, and transiently in the superficial inner retina. Retinal structure was preserved except for changes in areas of retinal detachment. No necrosis or destruction of the retina was seen. Immunohistochemistry showed no evidence of cytomegalovirus infection. Inoculation of culture medium containing fetal calf serum caused a similar reaction. It is concluded that vitreous and retinal inoculation of HCMV in the rabbit eye caused nonspecific inflammation without evidence of infection, so this is not a suitable model for HCMV retinitis.