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RESULTS
Retinal Architecture and Function Unaffected by AG1295 or AG1296 in Rabbit Eyes To assess AG1295 and AG1296 as a preventive drug for PVR in vivo, we evaluated if intravitreal injection of these drugs affects the retinal morphology or produces functional changes. We evaluated the retinal architecture histologically at 14, 21, and 28 days after the first injection of 100μmol/L of AG1295 or AG1296 (Figure 1). Compared with control eyes, no significant morphologic damages were observed at any time points.
The ERG data are shown in Figure 2. The bwave ratio of the rabbits treated with 100μmol/L of AG1295 or AG1296 was well preserved even with repeated injections for 28 days, indicating that both drugs had no adverse effects on retinal function and architecture.
AG1295 and AG1296 Inhibition of RPE Proliferation from PDGFAA and BB Proliferation of RPE after stimulation either with PDGFAA or BB is shown in Figure 3. The 50μmol/L concentration of AG1295 inhibited PDGFAA and BBinduced cell proliferation by 51% and 63% (P<0.01 for both, twotailed ttest), and the 50μmol/L concentration of AG1296 by 80% and 52% (P<0.01 for both, twotailed ttest) respectively. DMSO also inhibited cell proliferation, but when its concentration decreased to 1g/L, its negative effect is less than 14% (P>0.05) and can be ignored.
AG1295 and AG1296 Attenuated PVR in an Animal Model Figure 4 showed the percentage of tractional retinal detachment (TRD) in the rabbits that received RPE and PRP, followed by AG1295 or AG1296 or the control DMSO injection every week. The PVR scores were lower in the rabbits treated with AG1295 or AG1296 compared with controls. On day 3, in both AG1295 and AG1296 group, 2 of 18 (11%) treated eyes developed TRD, while 7 of 17 (41%) eyes developed TRD in control group. On day 7, in both treated groups, 3 of 18 (17%) got TRD, while the number of control group is 9 of 17 (53%). On day 14, TRD developed in 6 of 18(33%) AG1295 treated eyes, 5 of 18 (28%) AG1296 treated eyes and 12 of 17(71%) control eyes. After day 14, the AG1295 treated group also tended to develop PVR. The incidence of TRD on day 21 and 28 in AG1295 group was 11 of 18(61%) and 12 of 18 (67%) respectively, while the percentage of control group was 13 of 17 (76%) and 13 of 17(76%) respectively. The AG1296 group continually showed inhibitive effect on PVR in the late stage, and its incidence of TRD on day 21 and 28 was 5 of 18 (28%) and 6 of 18(33%) respectively. Therapeutic effect of AG1295 for rabbit PVR only showed in the early stage, and there is no difference in the incidence of TRD between AG1295 group and control group after day 14. However, compared with AG1295, AG1296 showed longer and stronger inhibitive effect for PVR, there was statistically significance difference in the incidence of TRD between AG1296 group and control group within the observed period.
DISCUSSION
Tyrosine kinase plays an important role in cell proliferation and differentiation. However, inhibitors of tyrosine kinase still cannot be used clinically to treat PVR because nonselective tyrosine kinase inhibitor may affect normal cell proliferation and other functions and thus could be potentially harmful to the retina and choroids. We had found AG1295, a specific inhibitor of PDGF receptor tyrosine kinase could attenuate the development of rabbit PVR without significant adverse effects on retinal function and architecture. Because AG1295 mainly block PDGF β receptor tyrosine kinase, so its inhibitive effect is limited in the pathological proliferation stimulated by PDGF, but its therapeutic effect only showed in the early phase on rabbit PVR. Namely, this drug could prevent the formation of fibrosis membranes, but once the membranes or strands appeared, it could not antagonize their contraction. We postulated that this phenomenon might reflect the role of PDGF in the development of PVR, because animal and clinical studies showed that PDGF has a negative correlation with the severity of PVR, i.e. the expression of PDGF decreased with the progression of PVR. In the present study, we found that a specific inhibitor of PDGF receptor could persistently prevent the development of PVR in rabbits. PDGF has two kinds of receptors: α and β PDGF α receptor can bind with either A or B chain of PDGF ligands, while PDGF receptor only with B chain [2]. AG1296 and AG1295 are specific inhibitors for PDGF α and β receptors respectively. Although both chemicals could restrain RPE proliferation and fibrosis membrane formation at PVR early stage, AG1296 showed a longer and stronger inhibitive effect than that of AG1295. This phenomenon indicated the following possibilities: 1) PDGF α receptor is related to PVR more closely than PDGF β receptor; PDGF may stimulate PVR by PDGF α receptor mainly. 2) Although the expression peak of PDGF is within the proliferative stage, PDGF α receptor may also intervene the contraction of fibrosis membrane. Many studies have suggested that TGFβ is the principal factor that mediates membrane contraction in PVR. However, Ikuno et al [10]proved recently that contraction of fibroblast induced by TGFβ depended on the expression of PDGF α receptor. Our results also showed that once the PDGF α receptors were inhibited, contraction of fibrosis membrane would be prevented and furthermore the incidence of TRD decreased. Although more researches are needed to reveal the mechanism of PVR and the animal model of PVR cannot be identical to human PVR, the present study suggested that inhibition of PDGF α receptor could be an effective therapeutic method for PVR.
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