Purpose: The goal of this study was the clinical evaluation of the neuroprotective properties of a dietary supplementation of forskolin, rutin and vitamins B1 and B2 on retinal ganglion cell activity. To this purpose, we focused both on IOP and the Pattern Electroretinogram (PERG). PERG is known to be significantly altered in glaucoma, and a decrease of PERG signal amplitude is an early sign of glaucoma progression.

Methods: 45 patients affected by POAG were evaluated in this study. Patients were randomly divided into 2 groups: Group A: 30 patients (60 eyes) with IOP compensated by topical pharmacological treatment, and taking the neuroprotective food supplement. Group B: 15 patients (30 eyes) treated with hypotonizing drugs alone. IOP and PERG were evaluated in both groupsat enrolment (T0), and after 2 (T1), 4 (T2) and 6 (T3) months.

Results: In group A, IOP significantly decreased from 15.6 mmHg (T0) to 14.6 (T1), 14.5 (T2) and 14.6 (T3) mmHg, while no differences were found in the control group B. As concerns PERG, while latency remained unchanged in both groups, we observed in group A – but not in group B – a progressive increase of amplitude from 3.4 µV (T0) to 3.6 (T1), 4.3 (T2), and 4.8 (T3) µV, achieving statistically significant differences at T2 and T3.

Conclusion: The results of the present clinical pilot study suggest a short-term neuroactive effect of the food supplement, as evidenced by the improvement of PERG amplitude in treated, but not in control patients.

Sisto D, Lavermicocca N, Errico D, Rusciano D (2013) Oral Administration of Forskolin and Rutin Contributes to Reduce Intraocular Pressure and Improve PERG (Pattern Electroretinogram) Amplitude in Glaucomatous Patients. JSM Biotechnol Bioeng 2(1): 1036.

•    Glaucoma
•    IOP
•    Forskolin
•    Neuroprotection
•    Pattern electroretinogram

POAG: Primary Open Angle Glaucoma; RGC: Retinal Ganglion Cells; IOP: Intraocular Pressure; BDNF: Brain-Derived Neurotrophic Factor; CNTF: Ciliary Neurotrophic Factor; TrkB: Tropomyosin-Related Kinase B; cAMP: Cyclic Adenosine Monophosphate; AH: Aqueous Humor

Primary Open Angle Glaucoma (POAG) is a progressive neuropathy of the optic pathway, characterized by loss of Retinal Ganglion Cells (RGC), cupping of the optic nerve head and visual field deficits [1,2]. Elevated Intraocular Pressure (IOP) is the main risk factor for development and progression of POAG [3], although it is no longer considered a peculiar characteristic of the disease [4–8]. Impaired ocular blood flow is another possible cause contributing to glaucoma development and/or progression, especially when IOP is apparently normal [9].

It is increasingly documented that a neurodegenerative cascade occurs in all POAG patients, even those with normal IOPs [10]. Neurodegeneration in glaucoma is related to progressive death of retinal ganglion cells (RGCs) and their axons, resulting in optic nerve atrophy and, ultimately, vision loss [11-17]. Histological studies have described a selective loss of RGCs and their axons in glaucomatous patients and roughly 1/3 of RGCs is already lost, when visual field defects become apparent in automated visual field testing [18].

Without some form of intervention, RGC death in glaucoma is fatally progressive [13,14]. Therefore, a synergistic therapeutic approach appears necessary, aimed not only at reducing IOP, but also at avoiding progression of the neurodegenerative events.

Similarly to other neurodegenerative disorders, several factors contribute to neurodegeneration in patients with glaucoma. Many authors have reported that disruption of axonal transport of neurotrophins, caused by either vascular ischemia or by mechanical compression, occurs in glaucoma [17,19]. Neurotrophins are essential for cell survival and axon regeneration, and RGCs fail to regenerate in their absence [20,21]. RGC loss can be delayed by application of neurotrophic factors such as Brain-Derived Neurotrophic Factor (BDNF) or ciliary neurotrophic factor (CNTF) [22,23].

Several studies have shown that the binding of BDNF to its receptor tropomyosin-related kinase B (TrkB), is important for RGC survival and axon regeneration [24,25], but BDNF cannot promote long-term survival in vivo, because of the downregulation of TrkB further to cognate ligand binding [26,27]. In this regard, the elevation of cyclic adenosine monophosphate (cAMP) can enhance RGC response to BDNF, thus promoting survival and axon growth [28,29].

Nonetheless, IOP still remains the main, if not the only, target for the pharmacologic treatment of this disease [3,30,31]. The first therapeutic approach still relies on the administration of hypotensive eye drops, with the intent of lowering IOP to a target value generally 20%–30% lower than the presenting value [6]. However, some patients may either initially be, or become, unresponsive to some of these drugs so that different combinations of them are required to maintain IOP at the desired value [7,8].

Forskolin is a natural compound extracted from the roots of the Indian Coleus plant (Coleus Forskohlii) [32]. It is a receptor-independent adenylate-cyclase activator, so that the concentration of the second-messenger cAMP rapidly increases in forskolin-stimulated cells [33,34]. Human non-pigmented ciliary epithelial cells have been shown to contain adenylatecyclase (subtypes II and IV), the activity of which contributes to the regulation of aqueous humor (AH) dynamics [3]. Forskolin treatment results in a reduced rate of aqueous secretion (and, consequently, IOP) from the ciliary epithelium in response to activation of adenylate-cyclase [30,31]. that may explain the hypotensive effect of topical administration of forskolin, as shown in humans [30] and experimental animals [35,36]. Moreover, forskolin-mediated activation of cAMP may contribute to neuronal cell survival and growth, by stimulating the paracrine circuit involving the neurotrophin BDNF and its receptor TrkB [37-40].

However, glaucomatous neurodegeneration is a multifactorial process that includes oxidative stress as a main contributor in the promotion of apoptotic RGC death [14,15].

In this regard, flavonoids are a large family of compounds derived from vegetables and fruits [41], which have shown antioxidant and neuroprotective activities in different neurodegenerative disorders [42,43]. In particular, some studies have described the potential protective effects of flavonoids on RGC degeneration [44,45].

Rutin is a natural bioflavonoid that has been shown to improve ocular blood flow and electroretinogram recovery in experimental glaucoma models [32,33]. It has a free radical scavenging activity and may thus protect sensitive cells from ischemia/reperfusion damage [34], as may happen during retinal vein occlusion, or suddenly elevated IOP.

It is known that vitamin B1 and B2 deficiency can affect the tissues of the eye. The recent Rotterdam study, a prospective population-based cohort of 3502 participants, showed that a low intake of retinol equivalents and vitamin B1 appears to be associated with an increased risk of open-angle glaucoma [46].

Vitamin B2 deficiency is associated with cataract [47]; this vitamin preserves levels of glutathione, the major endogenous antioxidant which, in turn, neutralizes detrimental compounds such as free radicals and reactive oxygen, that, as mentioned above, are implicated in apoptotic dell death of RGCs [14,15].

A food supplement is commercially available in Italy, containing 150 mg of coleus forskohlii extract titrated at 10% in forskolin, and enriched with rutin (200 mg) and vitamins B1 (0.55 mg) and B2 (0.7 mg) (Kronek®, SOOFT Italia). Such a food supplement would then deliver 15 mg of forskolin in each tablet.

The goal of this study was to evaluate the neuroprotective properties of Kronek on RGCs in glaucoma. To this purpose, we focused on tonometry and Pattern Electroretinogram (PERG), a recognized clinical tool proposed to monitor progression of glaucomatous optic neuropathy [48] through the measurement of RGC bioelectric activity. It is known that PERG is significantly altered in glaucoma patients, even before changes in the visual field become apparent [48–51].

The research followed the tenets of the Declaration of Helsinki. Upon recruitment each patient gave informed consent. Approval was obtained from the local Ethics Committee.

Three different glaucoma centers participated in this open, prospective, and randomized case-control trial. A total of 45 POAG patients of both sexes, aged between 30 and 70 years, with IOP values ≤ 18 mmHg under therapy (beta blockers, prostaglandins, alpha-agonists or their fixed associations), and stable for at least 3 months, were included. Patients with other concomitant systemic pathologies (preventing them from taking the food supplement) or cataract, were excluded. Enrolled patients, with IOP compensated by topical therapy, took either the hypotonizing treatment plus the food supplement with Forskolin, Rutin and vitaminsB1 and B2, one tablet, twice daily for 6 months (group A, 30 patients, 60 eyes), or the hypotonizing treatment alone (group B, 15 patients, 30 eyes). As endpoints, we focused on IOP and PERG that were evaluated at enrolment (T0), and then after 2 (T1), 4 (T2) and 6 (T3) months with Retimax (CSO strumenti of talmici, Florence, Italy). At each visit, patients were asked to return the empty used boxes of the food supplement, and inquired about any side effects.

Inclusion criteria

• Caucasian patients aged between 30 and 70 years

• No systemic pathologies or cataract

• Visual Acuity ≥ 8/10

• Refractive defect between -5D e +3D

• Astigmatism not beyond ± 1D

• Normal corneal pachimetry (? 520 e ? 580 µm)

• IOP ≤ 18 mmHg under therapy, and stable for at least 3 months

• MD worse than -6 dB at enrolment

At enrolment, IOP of all patients was measured in the morning by Goldman applanation tonometry, and the average value of 3 consecutive measurements is reported (T0).

PERG was recorded simultaneously from both eyes using ISCEV standard for clinical pattern.

Electroretinography [52] using skin electrodes that overcomes some limitations of current standard PERG techniques with corneal electrodes [53], the reproducibility of which depends on both the operator’s skills and patient’s compliance [54]. Moreover, PERG waveforms recorded in individual eyes are not contaminated by the responses originating from the contralateral eye, as may happen with some corneal electrodes [55,56]. HK Loop electrodes were hooked into the lower fornix. HK Loops were folded so that the contact windows on otherwise insulated wire were positioned on the bulbar conjunctiva, about 5 mm under the limbus. HK Loop electrodes did not touch the cornea. To achieve this, the limbs of the HK loop diverged widely (15–20 mm) before entering the fornix. The lead was then taped to the cheek, while reference and ground electrodes were fixed on the ipsilateral temple and the forehead, respectively.

The pattern stimulus consisted of a horizontal black and white checkerboard with a check size of 0.8°, 30° square field visual angle, 95% Michelson contrast, 40 candles/m2 of mean luminance, reversed in counterphase at 2 Hz (4 reversals per second). Signals were band pass filtered (1–30 Hz), amplified (50,000 V/v), and averaged (200 sweeps). Subjects with undilated pupils were classified into a refractive group, refracted for the viewing distance (573 mm), and asked to stare on a target at the center of the stimulus.

Subjects were allowed to blink freely. Recording time was approximately 3 min. Sweeps contaminated by eye blinks or gross eye movements were automatically rejected over a threshold voltage of 25 microvolts.

Baseline PERG values were recorded at enrolment (T0), and new measurements made after 2 (T1), 4 (T2) and 6 (T3) months from baseline.

Statistics

The differences of PERG responses between groups were evaluated by a non-parametric statistics (Mann-Whiney). 

Changes in PERG responses within the same group with respect to baseline were evaluated by one-way repeated measures ANOVA. In all analyses, a conservative p value less than 0.05 was considered as statistically significant.

Patients’ demography and enrolment values are summarized in Table 1. No significant differences (p≥0.05) in PERG responses or IOP values were observed at baseline between groups, indicating their homogeneity. Control patients happened to be of somewhat younger age than treated patients, but this is not expected to influence progression or compliance to treatment. No cases of intolerance or any other side effects due to the food supplement intake were reported, and no drop-off of patients occurred: all the enrolled patients completed the study protocol.

Figure 1a shows that the IOP decreased in group A patients from 15.6 mmHg (T0) to 14.6 (T1), 14.5 (T2) and 14.6 (T3) mmHg. The difference at each further time-point was statistically significant with respect to T0. No such differences were evident for the control group B (Figure 1b).

Figure 2 illustrates the variations of latency and amplitude PERG values for both treated and control groups. Group A patients showed a stable P50 peak time, whereas the P50-N95 peak amplitude increased from 3.4 µV (T0) to 3.6 (T1), 4.3 (T2), and 4.8 (T3) µV, achieving statistically significant differences at T2 and T3 (Figure 2a). In group B, there was no statistically significant trend of improvement (Figure 2b) of the P50-N95 amplitude.

RGC degeneration is the key hallmark of POAG [11–18]. Therefore, a major goal of neuroprotection for POAG treatment is the prevention or at least the delay of RGC death through the use of specific agents that might rescue and/or promote regeneration of already compromised RGCs.

In this paper, we used PERG analysis to obtain a direct and objective measure of RGC activity. In fact, it is known that PERG is sensitive to early RGC dysfunction caused by ocular hypertension and early glaucoma even before changes of the ONH structure or the visual field tests are apparent [48–51].

The obtained results of this clinical pilot study clearly indicate that oral treatment of tonometrically compensated POAG patients with an association of forskolin, rutin and vitamins B1 and B2 was able to further decrease IOP (Figure 1a) and improve, in the short-term (3 and 6 months), the PERG amplitude signal (Figure 2a), as shown by the increased P50-N95 amplitude.

None of the above effects were ever evident in the control group taking the hypotonizing treatment alone (Figures 1b and 2b).

The treatment group showed no differences in the latency of the P50-N95 wave (Table 1), in accordance with several literature data reporting that it is mostly the PERG amplitude that is significantly reduced in early glaucoma [57–59].

Accordingly, Parisi et al. correlated the improvement of the PERG signal after treatment of POAG patients with the potential neuroprotective compound cytidine-5’-diphosphocholine (citicoline), to the improvement of their retinal function [60].

The effect of forskolin could be truly neuroprotective, considering that all the patients had good IOP control (< 18 mmHg) before entering the study, and that the further IOP-lowering effect, around 1 mmHg, (Figure 1a), although statistically significant, should not be considered clinically relevant, according to the AGIS data [61]. Indeed, these effects on IOP were expected, on the basis of several clinical reports already showing the complementary effects of forskolin treatment with different hypotonizing drugs, or their different associations [62,63].

This low but significant IOP decrease has little meaning by itself in the control of IOP and neurodegeneration, but is an important marker of the actual presence of the forskolin molecule in the eye, where it can exert its neuroprotective activity.

Since IOP is only one among a number of different risk factors, the beneficial effects of the forskolin/rutin association on RGC function, as evaluated by PERG recordings, may be due to several mechanisms.

Many diverse factors can contribute to glaucomatous neurodegeneration, among which is the disruption of the axonal transport of neurotrophins [17,19–23]; the neuroprotective activity of forskolin could probably involve the modulation of neurotrophin/growth factor signaling, and in particular of BDNF signaling. Several studies have shown that the binding of BDNF to its receptor TrkB is important for RGC survival and axon regeneration [24,25]. Forskolin promotes the survival of retina neurons through the induction of BDNF mRNA, and of primary cultures of cortical neurons by stimulating the expression of both the full-length and truncated TrkB isoforms [64].

Moreover, forskolin has been shown to protect Neuro 2A neuronal cells from a surrogate of the organophosphate chemical warfare agents, soman and sarin [65], and to attenuate the adverse effects of long-term Schwann cell denervation on peripheral nerve regeneration in vivo [66], consistent with its property to modulate neurotrophin signaling.

It is known that forskolin raises levels of cAMP through the induction of enzyme adenylate cyclase [67,68], and the modulation of BDNF signaling could be mainly linked to this activity.

Embryonic and postnatal RGCs exhibited almost no responsiveness to peptide trophic factors unless their intracellular cAMP was simultaneously elevated [28] and BDNF does not promote long-term survival in vivo, as a result of the down-regulation of TrkB, following binding of BDNF [26,27]. In this regard, the elevation of cAMP can enhance the RGC response to BDNF for survival and axon growth [28,29]. cAMP increase, acting through the cAMP response element binding proteinactivated transcription factor, can potentiate the neurotrophin BDNF signal cascade either by blocking BAX/BAD pro-apoptotic signals or up-regulating anti-apoptotic BCL-2 expression [69], and can rapidly recruit TrkB to the plasma membrane of CNS neurons [39].

As for the neuroprotective activity of rutin on RGC function, it is reasonable to suppose it involves oxidative stress control. Oxidative stress has been implicated in many types of ocular diseases and can contribute to glaucomatous neurodegeneration, causing apoptotic cell death of RGCs [14,15]. Rutin has been shown to possess a strong antioxidant activity [70-73], and to be a good neuroprotective agent in vitro and in vivo [74-76]. In a recent work, rutin was demonstrated to protect RCGs from hypoxia-, glutamate- or oxidative stress, inhibiting both apoptotic and necrotic pathways, by the suppression of caspase-3 and calpain activities, respectively [77]. Along the same line, Falsini et al. reported a modest beneficial effect on PERG amplitude values of another flavonoid with antioxidant properties, epigallocatechin gallate, given for 3 months as dietary supplementation to POAG patients [78]. Moreover, rutin has vasoactive properties and improves ocular blood flow [79,80].

In conclusion, the results of the present clinical study suggest a neuroactive effect of forskolin and rutin supplementation evidenced by PERG analysis on RGC function.

To date, IOP is still the main risk factor contributing to the initiation and progression of glaucoma, but in several cases, glaucoma may continue to progress despite good IOP management. Therefore, neuroprotection appears to be a valid complementary strategy to treat glaucoma patients. A better and deeper understanding of the molecular mechanisms involved in the control of the IOP and the survival of RGC, as detailed in the discussion above, should help in identifying new targets for both a more efficient IOP control and for neuroprotection.

However, long-term clinical studies evaluating the progression of glaucomatous disease with standard diagnostic procedures also including visual field analyses are needed to confirm the potential use of forskolin and rutin supplementation as neuroprotective agents to complement the hypotensive therapy of glaucoma.

The authors wish to thank Dr Antony Bridge wood for critical proofreading of the manuscript.

Conflict of interest

D. Sisto, N. Lavermicocca, D. Errico declare that no competing financial interests exist. D. Rusciano is an employee of Sooft italia SpA, the producer of the food supplement used in this study.

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