Anti-vascular endothelial growth factor (anti-VEGF) agents have revolutionized the management of retinal vascular disorders characterized by pathological neovascularization or abnormal vascular permeability (Andreoli and Miller, 2007); hence, they have emerged as the first-line therapies for conditions such as diabetic macular edema (DME) and macular edema (ME) secondary to retinal vein occlusion (RVO). Conbercept is a fully humanized soluble VEGF-binding Fc fusion protein that exhibits high affinities for VEGF-A, VEGF-B, and placental growth factor; these are attributed to its unique structures of the extracellular domain 2 of vascular endothelial growth factor receptor 1 (VEGFR1) and extracellular domains 3 and 4 of VEGFR2, which enable more efficient reduction of vitreous VEGF levels and inhibition of neovascularization (Chen et al., 2013; Sun et al., 2017; Liu et al., 2022; Liu et al., 2019).

As outlined in the latest guidelines on clinical practices, panretinal photocoagulation (PRP) may be more suitable as an initial therapeutic intervention for patients with proliferative diabetic retinopathy (PDR), particularly under poor compliance conditions (Jampol et al., 2020), and remains the primary treatment in ischemic central retinal vein occlusion complicated by new vessels in the iris or angle (Scott et al., 2020; Nicholson et al., 2022). A combination of PRP and anti-VEGF agent could be a more optimal strategy in such conditions (Nicholson et al., 2022; Figueira et al., 2018), and more than one-third of ophthalmologists favor a combined strategy in clinical practice (i.e., real-world settings) (Singh, 2018; Singh and Hahn, 2019). However, there is no consensus regarding the optimal order in combined treatment. Although anti-VEGF administration prior to PPR could favor relief from ME, prompt recovery of visual acuity, and avoidance of ME severity induced by PRP, some practitioners choose to commence PRP prior to anti-VEGF for exuberant iris or angle new vessels in RVO or any imminent vitreous hemorrhage in severe PDR (Filho et al., 2011; Messias et al., 2012). Some clinicians opt to administer anti-VEGF agents at the end of vitrectomy combined with endolaser photocoagulation for PDR to reduce postoperative vitreous hemorrhage (Ahn et al., 2011; Sun et al., 2023). Thus far, only three retrospective studies have explored the order of anti-VEGF therapy and PRP in the treatment of DME secondary to severe non-proliferative diabetic retinopathy (NPDR) or PDR (Kartasasmita and Harley, 2019; Zhang et al., 2021; Alsoudi et al., 2024); however, these have yielded somewhat divergent results, and the underlying mechanisms remain to be elucidated (Zhang et al., 2021).

After intravitreal administration, drug elimination occurs via blood–ocular barriers and aqueous humor outflow to systemic circulation. High amounts of VEGF-binding Fc fusion protein were detected in the retinal and choroidal tissues after intravitreal injection into healthy rabbit eyes, indicating involvement of these tissues in the clearance route of VEGF-binding reagents (Joo et al., 2017). The endothelia of the retinal capillaries (inner blood–retinal barrier or inner BRB) and retinal pigment epithelium (RPE; outer blood–retinal barrier or outer BRB) are major barriers to macromolecule elimination from the vitreous space (Pitkänen et al., 2005; Del Amo et al., 2017). Retinal laser photocoagulation might open new pathways in the outer BRB for fluid and molecule transportation between the retina and choriocapillaris (Wallow, 1984; Peyman et al., 1971a, 1971b; Peyman and Bok, 1972). Moreover, scatter photocoagulation could increase leukocyte rolling in the retina, which is known to be involved in the breakdown of the inner BRB, resulting in retinal thickening and ME (Nonaka et al., 2002; Er et al., 2000; McDonald and Schatz, 1985; Tsujikawa et al., 1999). Deissler et al. (2020) observed that increased amounts of aflibercept pass through a monolayer of VEGF-A-challenged immortalized bovine retinal endothelial cells, likely owing to enhanced paracellular flow. It is not yet clear whether the drug clearance of VEGF-binding Fc fusion protein from the vitreous space is affected under impaired BRB conditions after scatter laser photocoagulation. In the present study, we assessed the distribution of intravitreally administered conbercept in the ocular tissues of non-photocoagulated and photocoagulated rabbit eyes to further analyze the impacts of photocoagulation on conbercept from the perspective of pharmacokinetics.

The pharmacokinetic parameters in different matrices were calculated using the sparse sampling computations of non-compartmental analysis available in Phoenix WinNonlin (version 8.1; Pharsight Corporation, Mountain View, CA, United States).

An independent-sample t-test was performed using SAS (Version 9.4; SAS Institute, Inc., Cary, NC) to compare the exposure pharmacokinetic parameters, including peak plasma concentration (C_max) and area under the concentration–time curve from time 0 to the last sampling time (AUC_0-t), for conbercept in the control versus photocoagulated groups. A two-way ANOVA was performed using GraphPad Prism (Version 10.1.2) to compare the decreases in conbercept concentrations with time in the different matrices. Given the sampling limitations and poor linear fit in the elimination phase of the concentration–time curve, statistical comparisons of the elimination half-life (t_1/2), clearance (CL), volume of distribution (V), and mean residence time (MRT) between the two groups could not be performed.

PRP and anti-VEGF therapy are the treatment options for patients with ischemic retinopathy, including PDR, severe NPDR, and RVO complicated by neovascularization (Scott et al., 2020; Figueira et al., 2018; Gross et al., 2015). Different guidelines recommend that anti-VEGF injection be combined with PRP in certain indications (Jampol et al., 2020; Scott et al., 2020; Nicholson et al., 2022). As shown by the ASRS PAT surveys, combined therapy for the treatment of PDR has gained popularity in clinical practice (Singh, 2018; Singh and Hahn, 2019). In particular, to the best of our knowledge, no studies have assessed the impacts of retinal scatter photocoagulation on the ocular pharmacokinetics of intravitreal Conbercept.

Li et al. (2012) and Di et al. (2021) investigated the ocular pharmacokinetics of normal rabbits following a single intravitreal injection of conbercept (0.5 mg). Their results showed that the peak concentrations in vitreous humor were 249.22 μg/mL after 0.5 d and 126.25 μg/mL after 1 d of intravitreal injection, but both studies reported a half-life of 4.24 d. In our study, we obtained a similar half-life for conbercept in the vitreous humor of the control rabbits (4.36 d), and the peak concentration of 135.10 ± 19.39 μg/mL was observed in the vitreous humor 1 d after intravitreal injection. The half-life of conbercept in the aqueous humor in our study was higher than that reported by Li et al. but lower than that noted by Di et al., which may be related to the different breed of experimental rabbits and ELISA used. The present study shows that scatter retinal photocoagulation (at 3 d) shortly before conbercept injection could lead to increased drug clearance and shorter half-life, resulting in less exposure to the vitreous humor, aqueous humor, and retina compared to the non-photocoagulation group. Substantial blood volume and rapid blood flow velocity are believed to be the reason for insignificant C_max values in the choroidal tissue.

Recently, Alsoudi et al. (2024) suggested that the sequence of treatment in combination therapy may impact patient outcomes in large-scale retrospective studies; they reported that patients treated with PRP prior to anti-VEGF injections had higher risks of vitreous hemorrhage, tractional retinal detachment, or progression to pars plana vitrectomy compared to those treated with anti-VEGF injections prior to PRP. In another small retrospective study, Zhang et al. (2021) noted no significant differences in the visual and anatomical outcomes in the compared groups but observed that PRP after intravitreal conbercept required fewer injections than prior PRP administration at 1 and 2 years follow-up. In addition to speculating that timely anti-VEGF administration can rapidly suppress VEGF activities in the retina and vitreous humor, reduce vascular permeability, and promote the resorption of intraretinal or subretinal fluid, we demonstrate significant differences in drug exposures following intravitreal conbercept administration in the present study along with prior scatter retinal photocoagulation treatment inducing higher drug clearance and shorter half-life of conbercept; these procedures would result in less powerful inhibition of VEGF and require more frequent drug injections as well as highlight the mechanisms underlying the distinct prognosis.

Thermal destruction by retinal photocoagulation triggers focal inflammation in the retinal tissue. Leukocyte rolling and inflammatory factors, including IL-6, VEGF, and TNFα, have been shown to increase after scatter laser photocoagulation in both the photocoagulated and untreated portions of the retina, resulting in augmented vascular permeability and retinal edema (Nonaka et al., 2002; Er et al., 2000; Masahiko et al., 2009; Itaya et al., 2007). VEGF and its combination with TNFα and IL-1β can disrupt retinal endothelial barrier integrity remarkably in vitro, inducing permeability to large molecules and even aflibercept (Deissler et al., 2020; van der Wijk et al., (2017)). We hypothesize that the enhanced permeability to conbercept within the inner BRB triggered by abundant inflammatory factors following scatter photocoagulation may contribute to the observed increase in drug clearance. Additionally, neonatal Fc receptor (FcRn) expression has been proven to be upregulated in laser-photocoagulated rat retina, potentially mediated by TNFα (Kim et al., 2009), while the role of FcRn in the uptake and transport of Fc-containing molecules has been firmly established in RPE and retinal endothelial cells (Dithmer et al.,2016; Deissler et al., 2017). Accordingly, by integrating these two lines of evidence, one can excuse the possibility of a relationship between FcRn upregulation and lower conbercept exposure in scatter photocoagulation. Moreover, laser photocoagulation disrupts the external limiting membrane and RPE, thereby extending the outer nuclear layer into Bruch’s membrane and the choroid Fukuchi et al., (2001). Intravitreal peroxidase has been shown to diffuse rapidly through all layers of a 3-week-old photocoagulated scar in contact with the Bruch’s membrane (Peyman et al., 1971b). This raises the question of whether conbercept can freely traverse the compromised RPE barrier in a fresh photocoagulated spot. Hence, further research is warranted to elucidate the underlying mechanisms.

Irrespective of the findings, there are some limitations to our study. First, the interval between photocoagulation and intravitreal injection in our study may not fully replicate clinical practice. Owing to ethical considerations regarding animal welfare and the principle of minimal use, a 3-d research interval was selected herein given the pronounced laser-induced damage observed during this period. Second, the pharmacokinetics observed in rabbits may not be entirely comparable to those in humans given the anatomical disparities between rabbit and human eyes. Finally, normal healthy rabbit eyes were utilized in our study, and an animal model with ischemic retinopathy, such as diabetic retinopathy or RVO was not established; this is another avenue for further exploration in the future.