found a very good January 2014 paper that specifically compares Retigabine, Flupertine, and two other BK channel modulators on their effectiveness with regard to tinnitus:
http://www.ncbi.nlm.nih.gov/pubmed/24681057
It's not a public paper so I will try and summarize it for those that can't access it.
excerpts / interpretations:
"For retigabine, the EC50 (8.0 8μM) determined in this study was comparable, though slightly higher, than the EC50 for KV7 subtype current activation (0.6–6.2 8μM; nH roughly 1.0 for each subtype; Tatulian et al., 2001). Retigabine at this concentration range induced observable network inhibitory effects"
[The EC50 they got for Retigabine was 8μM. Interesting to point out that the Tatulian paper (
https://www.ucl.ac.uk/npp/research/dab/dablab/Tatulian_et_al_2001.pdf) had specific EC50 values for each channel that were lower.]
"Even at low concentrations (1.0–2.0 μM), rapid inhibitory effects on network spike and burst rates were observed for all four drugs, although they often required up to 15 min of stabilization. A decrease in spike and burst activity under retigabine and flupirtine at high concentrations ( > 10 μM) was almost instantaneous "
[pretty potent stuff]
"The stabilized activity levels (spike rate) recorded 1.0 h after the wash were quantified as percent of the reference activity before drug applications. Retigabine and flupirtine both exhibited full recoveries near 100%:"
[Even after application of 50μM solution, after the washout period the activity came back completely normal. This is good in the sense that there is no lasting damage to the cells (the other two BK channel modulators did not have full recoveries), but it also means that it is possible that there might not be lasting improvement, unless some secondary homeostatic plastic mechanisms bring the network back to the previous non-tinnitus state.]
"However, modulations of action potential duration, as reflected in peak maxima positions, followed a different pattern. Retigabine shortened the duration by 9.5715% (P<0.0001), while flupirtine did not cause a change ."
[from my understanding this would be a beneficial effect for the high frequency firing neurons in the auditory cortex].
on flupertine:
"In this study, we found that flupirtine was twice as potent in its inhibitory effect on the auditory cortical networks as retigabine (4 μM vs. 8 μM), and exerted significantly different effects on extracellular action potentials. This is perhaps due to other mechan- isms of flupirtine at comparable concentrations, such as the effect on the G-protein coupled inwardly rectifying K current at the EC50 of 0.6 μM (Jakob and Krieglstein, 1997), or the potentiation of GABAA current at<10 μM (Klinger et al., 2012). Expectedly, we observed a significant GABAergic effect of flupirtine, as the EC50 increased 6-fold under the presence of the GABAA blocker pentylenetetrazol, an effect of which was not apparent for retigabine."
[flupertine does some interesting stuff, but for our purposes is not as good as retigabine, and that is due to the Therapeutic dosage graph below]
[Nat is the natural rate of fire, PTZ, is the drug they used to simulate tinnitus, and the x axis shows the μM of retigabine (RTG) and flupertine (FPT) respectively. FPT has a pretty steep sigmoid curve, and doesn't show any activity till you get well past 5 μM. retigabine shows its effects almost immediately from 1 μM.]
finally:
"We estimated the therapeutic potential of K channel openers in suppressing tinnitus-like activity by quantifying the responses in the auditory cortical networks.
Retigabine had the highest therapeutic potential, with a therapeutic concentration of 7.4 μM – a concentration at which counteraction against induced- hyperactivity were effective – followed by NS1619 (15.2 μM), flupirtine (23.3 μM), and isopimaric acid (30.0 μM). These values were well within the range of their effective concentrations against epilepsy (3–100 μM; Kobayashi et al., 2008), thus,
possible off-label treatment for tinnitus is plausible (and safe). Clinical studies had calculated the free brain concentration of retigabine taken at 1200 mg/day to be around 2.0 μM (Large et al., 2012) "
[sounds pretty awesome to me!]
More thoughts on dosage:
The general consensus is the epilepsy is a genetic disease, so retigabine was intended to be taken for very long periods of time (years). I don't think that is the case with us, as tinnitus is not genetic and is probably just reversible hyperactivity of neurons, that may plausibly cured. I don't think the dosage schedule of retigabine for epilepsy is optimal. if you look at the data above, it is clear that a sufficient dosage is required to do any real work. Here is another graph from that paper:
I am of the opinion that 900mg/day is the very
least that someone should try. I don't see any reason not to try 1200mg/day, if it will get one to the 2μM brain concentration. There were some issues with taking this drug at very high dosages, I recall from the australian report that out of 6 healthy volunteers that took a one-time dosage of 900mg (in one sitting), 2 of them had some heart arrythmias that went away after a few hours (this might be the slight effect on Kv7.1 at very high dosages), but I am not advocating 900mgX3 = 2700mg/day, but at least something high enough to get some effect. And I don't think that taking this drug forever is a good idea, although if one were optimistic that a better solution will come around in the next 5 years, I suppose it would not be too big a deal if some people were deciding to take retigabine for the foreseeable future in anticipation of something better. But hopefully the effect of this drug at a high enough dosage will induce permanent reset and we can kiss tinnitus goodbye.]