The potential of mitochondrial FPF imaging in practice

     New Insights in Imaging     

Bios Ms. Pur is a medical student at Schulich School of Medicine and Dentistry, Western University, London, Ontario, and a junior scientist at the Octaine Imaging Lab. Ms. Sodhi is a medical student at the University of Cambridge, United Kingdom, and a senior scientist at the Octaine Imaging Lab. Dr. Choudhry is co-founder and medical director of Vitreous Retina Macula Specialists of Toronto and principal investigator at the Octane Imaging Lab. DISCLOSURES: Ms. Pur and Ms. Sodhi have no relevant disclosures. Dr. Choudhry disclosed relationships with Alcon, AbbVie, Apellis, Bayer, Carl Zeiss Meditec, Hoffman La Roche, Johnson & Johnson and Novartis.

The retina consists of highly metabolically active tissue, and consequently it’s susceptible to mitochondria-related damage.¹ Mitochondria play an essential role in maintaining cellular energy homeostasis, particularly at the level of the photoreceptors.² Numerous studies have demonstrated that mitochondrial dysfunction is a key factor in the pathogenesis and progression of retinal diseases, such as diabetic retinopathy,³ central serous retinopathy,⁴ age-related macular degeneration,^5,6 glaucoma,^7,8 retinal dystrophies⁹ and optic nerve pathologies.¹⁰ 

Mitochondrial metabolic dysfunction renders the neural retina and retinal pigment epithelium vulnerable to oxidative stress, which can be measured using mitochondrial flavoprotein fluorescence (FPF), also called fluorescence lifetime imaging. This novel, noninvasive imaging modality functions as a marker of oxidative stress and mitochondrial dysfunction by quantifying the ratio of oxidized flavoproteins to reduced flavoproteins in mitochondria.¹¹ 

Compared to spectral domain optical coherence tomography, the gold standard for diagnosis and monitoring of retinal diseases, which offers visualization of structural alternations, FPF provides information about subtle functional alternations that may precede structural changes.

Clinical utility of FPF imaging

Emerging studies highlight that FPF may be an indicator of retinal disease activity.¹² Specifically, it may serve as a tool for early detection, classification and prognostication of retinal diseases, as well as evaluate a patient’s response to therapy. FPF was shown to detect and classify disease stages of AMD into early AMD, intermediate AMD, advanced geographic atrophy and advanced neovascular AMD based on mean differences in flavoprotein fluorescence heterogeneity.^5,6 FPF has the potential to act as predictive marker of AMD disease activity, a promising idea given that little is known about a patient’s individual-level risk of AMD progression. 

Moreover, FPF has been shown to positively correlate with the degree of DR^3,12,13 and provide information regarding the therapeutic response to anti-VEGF that OCT may not capture.¹⁴ A pilot study of eight patients with DR and diabetic macular edema, in whom FPF was measured in the same eyes before and after anti-VEGF treatment, showed a significant negative correlation between improvement in best-corrected visual acuity and average FPF intensity values.¹⁴ 

Interestingly, the authors remarked that improvements in BCVA and increased FPF values (i.e., signaling improved mitochondrial function), but not OCT central macular thickness (CMT), occurred in anti-VEGF-treated patients with good vision and lower degrees of CMT.¹⁴ This suggests that FPF may evolve as a dynamic, early marker of treatment response, correlating with BCVA changes that precede those in OCT CMT.¹⁴ 

Potential in central serous retinopathy

Similar uses were reported in central serous retinopathy, specifically in a pilot study of three patients with CSR that reported detecting increased FPF average values as early as one week after symptom onset, compared with conventional FAF that detects changes several weeks after diseases onset.⁴ 

Furthermore, in a large cohort of 157 patients with inherited retinal dystrophies, mean FPF heterogeneity was found to correlate with FAF lesions, suggesting its relevance as a clinically relevant imaging marker.⁹ 

Other uses for FPF include disease detection and monitoring in optic nerve pathologies, open-angle glaucoma⁷ and expanding knowledge on neurodegenerative conditions such as Parkinson’s disease, Alzheimer’s and other forms of dementia.^15,16 

A side-by-side comparison of intraretinal imaging (top row) and mitochondrial flavoprotein fluorescence (FPF, bottom row) for various disease states: A) healthy control; B) early age-related macular degeneration; C) intermediate AMD; D) advanced geographic atrophy; and E) advanced neovascular AMD.

Advantages and disadvantages of FPF

FPF offers several benefits, such as noninvasively providing objective, functional information of early alternations in mitochondrial metabolism, which have been suggested to precede structural changes captured by OCT. However, the promise of this evolving technique must be considered in the context of several limitations. 

First, no universal standards exist in the type of devices used, image processing parameters and healthy FPF reference values. For example, some authors have reported on FPF signal intensity while others reported on FPF signal heterogeneity. Furthermore, the choice of image processing parameters isn’t generally justified, rendering the methodology and device features a “black box.”

Our understanding is also evolving that patient factors such as age, gender, the presence of a cataract,¹³ corneal pathology¹⁷ and ocular or systemic comorbidities increase the variability in FPF signal and consequently decrease the reliability and generalization of reported findings. 

We need larger studies that include well-defined populations, that clearly outline methodology and adjust for confounding factors following a standard protocol before we can widely use these devices in the clinic.

Future of FPF

FPF is emerging as a powerful yet little-known tool for the detection and classification of retinal disease, and for monitoring the therapeutic effects of treatment of retinal diseases. It adds a complementary way of interrogating and visualizing function of retinal processes. 

Perhaps one of the most exciting avenues for FPF is its potential use in conjunction with therapeutics that target mitochondrial dysfunction in ocular diseases.¹⁸ Other future directions include its use along with artificial intelligence-based analyses of ocular biofluids and OCT imaging to provide care tailored to individual patients.¹⁹ RS

REFERENCES

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^{19.  Pur DR, Krance SH, Pucchio A, Miranda RN, Felfeli T. Current uses of artificial intelligence in the analysis of biofluid markers involved in corneal and ocular surface diseases: A systematic review. Eye. Published online November 15, 2022. doi:10.1038/s41433-022-02307-9.}