Author Response: The Future of Retinitis Pigmentosa Research: Unlocking the Potential of Branched-Chain Amino Acid Therapy

We appreciate the thoughtful comments from Dr. Jiang and Dr. Mou¹ regarding our recent study on the efficacy and safety of branched-chain amino acids (BCAAs) in patients with retinitis pigmentosa (RP).² Their letter provided valuable insights into the future direction of RP research, including suggestions for refining study methodologies and exploring the potential mechanisms of BCAAs. 

We are grateful for their interest in our study and their constructive feedback. In particular, we appreciate the emphasis on the importance of incorporating the genetic background, disease stage, and additional imaging biomarkers into future research. Their suggestions for investigating the pharmacokinetics of BCAAs and the possible role in the mTOR pathway are insightful. These could assist in obtaining a deeper understanding of the therapeutic potential of BCAAs in RP. 

We have addressed each of the key points raised in the letter below. 

Patient Stratification

In this clinical trial, we limited the stratification factors to age and Humphrey visual field mean deviation (MD) values because of the relatively small sample size. As Jiang and Mou pointed out, we acknowledge that ensuring an even distribution of a heterogeneous disease, such as RP, is crucial. 

Regarding the genetic background, RP is associated with a wide range of causative genes. The presumed causative gene was identified in only 43% of the participants in our study and included nine different genes. Given this variability, it was not feasible to use genetic background as a stratification factor. Ideally, a trial that includes only patients with specific genetic pathogenic variants could be conducted. However, owing to the limited number of eligible patients, this option was not viable in our study. Future research would plan a multicenter collaborative clinical trial targeting patients with specific genetic pathogenic variants. 

For disease stage, we restricted enrollment to patients with an Humphrey visual field 10-2 MD range of −5 to −25 dB, thereby excluding both very early and very late-stage RP cases. Moreover, because the MD values were used as a stratification factor, we believe that this served as a broad categorization of disease stages. 

Additionally, we aimed to enroll patients with progressive visual field loss (defined as a negative MD slope) to ensure a relatively homogeneous population in terms of disease progression. However, because visual field tests are prone to fluctuations, confirming reliable disease progression rates requires a longer observational period at the same site, which makes it difficult to include patients who visited the hospital of the study center relatively recently. Furthermore, in usual clinical situations where most of the patients have a visual field tests once a year, the visual field often progresses to fairly advanced stages by the time that the visual field progression rates are confirmed. Additionally, during discussions with the Japanese regulatory authorities, we were advised not to impose strict inclusion criteria based on the disease progression rate. Our trial included a significantly heterogeneous population in terms of the baseline progression rate. We believe that this variability may have contributed to the lack of a statistically significant difference in visual field progression between the treatment groups. 

Regarding subgroup analyses, our data suggest that BCAA treatment is associated with a slow rate of visual field progression in certain subgroups, such as those with pathogenic variants in EYS gene or specific disease stages. However, owing to the small sample size and high variability, these differences were not statistically significant. 

Pharmacokinetics and Mechanistic Studies

The BCAA granules (TK98) used in our study were identical in composition and dosage to a formulation approved in Japan for patients with liver cirrhosis and hypoalbuminemia. Pharmacokinetic data on the plasma concentrations of L-leucine, L-isoleucine, and L-valine after oral administration are available in the official product information of the drug. When a single dose of TK98 was administered to fasting adult males, the maximum concentration values were reported as 31.0, 58.5, and 46.8 µg/mL for L-isoleucine, L-leucine, and L-valine, respectively.³ 

Although it is challenging to directly assess BCAA penetration into the human retina or its ability to cross the blood–brain barrier in vivo, previous studies using radiolabeled BCAAs in rats have detected these amino acids in the brain and ocular tissues.³ These findings suggest that BCAAs can cross the blood–brain barrier and reach the intraocular structures. 

Further pharmacokinetic analyses comparing BCAA plasma concentrations between patients who exhibited clear suppression of visual field progression and those who did not could have provided additional insights. Such an investigation may reveal potential correlations between systemic BCAA levels and therapeutic efficacy and inform future clinical trials. 

Additional Biomarkers and Imaging Modalities

As suggested, not only should we evaluate the effects of BCAAs, but we should also explore biomarkers, genomics, and metabolomics to monitor RP progression. This area of research is important and challenging, and we recognize its potential value in improving disease monitoring and treatment evaluation. We agree that incorporating quality-of-life measures (e.g., visual function-based scores) would also be a valuable approach. We are committed to addressing this issue in future studies as it may provide new insights into RP pathophysiology and enhance the development of more effective therapeutic strategies. 

Assessment of Retinal Morphological Changes Using Supplementary Imaging Biomarkers

In this clinical trial, we performed fundus photography at baseline and final visit, along with longitudinal optical coherence tomography (OCT). Therefore, as suggested by Dr. Jiang and Dr. Mou,¹ it is possible to analyze morphological changes, including optic disc cupping, retinal nerve fiber layer thickness, GCC thickness, and outer retinal thickness, between the treatment and placebo groups. We analyzed the central total retinal thickness, measured automatically using the Spectralis OCT software; a lesser decrease in the BCAA-treated group was observed, but did not attain statistical significance (Table). 

Table.

 

Total Retinal Thickness

 

Our previous studies demonstrated that BCAAs have the potential to protect ganglion cells not only in models of RP, but also in a mouse model of glaucoma.⁴ Given these findings, an additional analysis of imaging data from this study may help to clarify the effects of BCAA on the inner retinal layers as well as the outer retinal layers in humans. We appreciate this valuable suggestion and plan to explore these aspects in future studies. 

We acknowledge the need to establish a morphology-based evaluation system to quantify retinal structural changes. Through rigorous investigation, including iterative experimentation, we found a strong correlation between ellipsoid zone length variations and declines in visual function.⁵ Consequently, we incorporated ellipsoid zone length analysis as one of the secondary end points.² It is crucial for researchers worldwide to collaborate in developing a robust evaluation system based on imaging techniques such as OCT, fundus photography, and scanning OCT. Establishing a standardized morphology scoring system would enable the dynamic tracking of retinal structural changes over time, ultimately leading to more precise assessments of drug efficacy. 

In this clinical trial, we chose visual field test results as the primary end point because there was no precedent of accepting morphological parameters as the primary end point by the Japanese regulatory authorities. However, if a structural evaluation method that closely correlates with visual function can be established, using structural parameters in clinical trials could provide more objective and less variable data, making it more feasible to detect the efficacy of the study treatment. 

Potential Neuroprotective and Systemic Effects of BCAAs

We did not consider testing cognitive function or monitoring the functionality of the visual cortex initially; however, given BCAAs' potential neuroprotective properties, this idea is indeed fascinating and worth exploring in future studies. BCAAs are well-known for their role in muscle recovery among athletes, and their potential systemic effects on overall health are an intriguing area of investigation. 

For this study, we collected general blood test data, which were analyzed to assess safety and provide insights into the potential systemic effects of BCAA supplementation. We plan to analyze these data to explore any correlations that could support the hypothesis of broader neuroprotective or metabolic benefits. 

We express our sincere gratitude for the insightful and suggestive comments. Although gene therapy is becoming increasingly practical, it is not universally applicable to all genetic mutations. Furthermore, for patients with undetermined genetic causes, the development of neuroprotective treatments that can prevent disease progression at any stage is highly anticipated. We hope that researchers worldwide will continue to collaborate to develop better evaluation systems, which will eventually lead to advancements in treatment options for retinal diseases such as RP. 

Acknowledgments

Disclosure: H.O. Ikeda (P); T. Hasegawa, (P)