Pharmacogenetic Labyrinth of Neovascular Age-Related Macular Degeneration Therapy: How to Escape and Move Forward?

KEYWORDS::

• age-related macular degeneration therapy
• bevacizumab
• polymorphisms
• ranibizumab
• VEGF

Neovascular age-related macular degeneration (AMD) is characterized by strong neoangiogenic phenomena, which are regulated by numerous growth factors. Among them, VEGF-A is a main actor of this complex and multistep process. In recent years, the coming of new drugs, which are able to antagonize the VEGF-A protein (i.e., pegaptanib, bevacizumab and ranibizumab), has significantly changed the prognosis of the disease with a significant improvement of best corrected visual acuity in all subtypes of choroidal neovascularization [1,2]. However, a more detailed analysis of individual results showed that approximately 75% of ranibizumab-treated patients achieved an improvement in visual acuity, whereas the remaining 25% patients got worse [1,2]. Why do patients similar for clinical stage, gender, race and age, show so different responses to the same treatment?

The pharmacogenetic labyrinth

To provide an actual answer to the individual variability in response to anti-VEGF treatment, researchers have focused their efforts in the direction of pharmacogenetic characterization of the therapies for neovascular AMD. In fact, possible correlations between clinical response to anti-VEGF treatments and SNPs of several genes have been demonstrated, even if with conflicting results [3–13].

The CFH gene has been widely investigated in relation to both photodynamic therapy with verteporfin and anti-VEGF drugs [3,6,11–13]. A recent meta-analysis of over 1500 patients showed that the clinical response to anti-VEGF drugs is 1.6-times lower in patients carrying the CFH rs1061170 CC genotype compared with patients carrying the CFH rs1061170 TT genotype (p = 0.020) [9]. On the other hand, data from the CATT study, a prospective clinical trial with more than 800 enrolled patients, did not show any association between the CFH rs1061170 genotypes and the functional response to anti-VEGF (i.e., ranibizumab and bevacizumab) [5]. Numerous VEGF-A SNPs have also been largely investigated in patients treated with ranibizumab or bevacizumab. The researcher‘s interest has been mainly focused on polymorphisms of the promoter region and, in particular, on VEGF-A rs699947 [4,8,10]. Lazzeri and colleagues have showed a positive response to ranibizumab in patients carrying the C allele with a significant gain of best corrected visual acuity [4]. On the contrary, no significant correlations between VEGF-A rs699947 genotypes and ranibizumab have been found by other authors [8,10]. A combined analysis on VEGF-A, CFH and ARMS2 SNPs has shown that the presence of all six risk alleles results in a reduction of visual acuity compared with patients without mutated alleles after ranibizumab injections [12]. Also, other VEGF-A SNPs have been studied with varying fortunes: Abedi et al. have found a positive response to ranibizumab or bevacizumab in patients carrying the rs3025000 T allele, but no association for the other polymorphisms investigated [7], while no significant association were found in the works of other authors [8,10]. Although this great heterogeneity of results can be partially explained by the use of different drugs and regimens of administration, as well as different evaluation criteria, sample size and follow-up, at the present time we can assert that there is not a well-defined genetic marker that is able to predict indubitably the response to current available treatments. But why can the identification of a specific genetic marker be so important in the management and treatment of neovascular AMD?

Importance of a genetically personalized therapy

As already observed in many other fields of medicine, genetic background may influence an individual‘s response to treatment. The personalization of therapy is a concept that seems to adapt perfectly to the treatment of neovascular AMD. In fact, fundoscopy presentation of neovascular membrane represents only the final result of a complex cascade of events that can be modulated at different steps and, therefore, they may require different treatments or doses from patient to patient. The most plausible pathophysiological hypothesis for changing treatments or doses could be related to the different levels of vitreous VEGF-A in diverse patients. This hypothesis is supported by studies that have found higher VEGF vitreous concentrations in patients with neovascular AMD compared with healthy controls [14,15], and higher VEGF vitreous concentrations being associated with a worse prognosis [16]. Strangely, to date there are no data available on possible correlations between SNPs and VEGF-A vitreous concentrations, except for the work of Lin and colleagues [17]. These authors looked at the vitreous VEGF-A levels of six wet AMD patients carrying the VEGF +936TT genotype (44.77 ± 12.21 pg/ml), comparing the results to the observations from five normal controls carrying the CC genotype (11.68 ± 3.58 pg/ml). The differences were statistically significant but the sample size was too limited for any possible definitive conclusion [17]. The genetic variability in the expression of the VEGF-A protein may be the cause of a reduced or absent response to the anti-VEGF treatment. In fact, anti-VEGF drugs work by blocking the VEGF-A protein. In the presence of higher vitreous VEGF-A concentrations, the standard dose of the commercially available anti-VEGF drug could not completely neutralize the vascular growth factor.

How can we use this information in clinical practice? The SAVE study has shown that patients with neovascular AMD refractory to standard anti-VEGF doses (ranibizumab 0.5 mg/0.05 ml) achieved both anatomical and functional improvement with higher intravitreal concentrations of the same drug (ranibizumab 2.0 mg/0.05 ml) [18]. However, despite the good results obtained, this clinical trial is burdened by an important limitation: the enrolled patients were probably treated for months with an inadequate dose, developing a severe distress of photoreceptors. Thus, the functional recovery was limited (+3.3 ETDRS letters at 3 months) [18]. In fact, if the intra- and sub-retinal edema is maintained for longer, the distress of the photoreceptors will be greater. In addition, the quality of scar tissue is another key aspect for the final prognosis: if a proper and earlier treatment is administered (e.g., by increasing the dose or by reducing the time interval between injections of anti-VEGF drugs), a smaller scar in the retinal tissue will be found, resulting in a better functional recovery.

Based on these premises, the early identification of the subset of nonresponder patients (e.g., those patients with genotypes or haplotypes that are compatible with high concentrations of vitreous VEGF-A) is urgently needed because it would be possible to administer personalized doses or to follow proper regimens of administration from the first injection, preserving visual acuity and reducing the number of treatments and costs for national health systems. Obviously, in addition to this, the early identification of those who respond to therapy could be extremely important. To date, all the protocols of anti-VEGF drug administration (i.e., Pro Re Nata protocol or monthly protocol) provide for a loading phase of three consecutive monthly injections. However, some patients immediately show an optimal response after just a single injection of the anti-VEGF drug. Thus, the loading phase may not always be necessary, reducing the related surgical risks and costs. Finally, there is another important aspect that ophthalmologists can see in daily practice. Some patients present a gradual loss of efficacy of anti-VEGF drugs. Why? Is there a drug resistance to anti-VEGF drugs? If yes, could it be associated with a specific genetic background such as a particular VEGF-A SNP? This is quite a new concept in ophthalmology, but oncological experience teaches us that is a possible occurrence [19]. Indeed, although no conclusive data are currently available on the modulation of pro- and anti-angiogenic factors in the vitreous after anti-VEGF drug injections, the clinical drug resistance could be reasonably determined by upregulation of proangiogenic factors other than VEGF-A and/or downregulation of antiangiogenic factors such as thrombospondins. On this basis, prospective clinical trials to evaluate changes in vitreous concentration of pro- and anti-angiogenic factors after anti-VEGF drug injections in genotyped patients are highly necessary and may provide surprising results. Moreover, recent work by Leveziel and colleagues seems to suggest that immunization against ranibizumab could be observed and may influence the clinical response [20]. May genetic background be responsible for this effect? Further clinical studies are needed to investigate these possible scenarios.

Which is the right path to get out of this labyrinth?

To date, the countless available genetic and pharmacogenetic data on AMD have probably generated more confusion than certainty among ophthalmologists, even in the clinical management of patients. In fact, despite the relevant efforts completely focused on the finding of a genetic marker to predict the risk of AMD or the response to anti-VEGF drugs or photodynamic therapy with verteporfin, in our opinion, other fundamental elements have been neglected for the personalization of AMD therapies. A key point that should be investigated is represented by the comprehensive knowledge of the phenotypic expression of the various SNPs, especially focusing on the VEGF-A gene. Indeed, standard therapy for neovascular AMD is represented by anti-VEGF drugs; therefore, the real first step toward a personalized therapy should be the evaluation of the VEGF-A gene expression in the vitreous of patients, and then measuring the VEGF-A concentrations in order to associate them to a precise genotypic or haplotypic background. In conclusion, the certain identification of an association between the VEGF-A genotype or haplotype and the VEGF-A phenotype is the prerequisite for any further study to establish effective anti-VEGF treatment regimens and dosing outsets.

Financial & competing interests disclosure

The authors have no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.

No writing assistance was utilized in the production of this manuscript.