Ki value or relative binding potency as ligand for Loperamide?

[Nagelfar]

Apparently since nano-particles have been used to show efficacy at Loperamide crossing the BBB ( http://www.ncbi.nlm.nih.gov/pubmed/9098875 )

Shouldn't it follow that I'd be able to find some ratio of Loperamide's binding strength to the opioid receptors relative to morphine, as one can find with most any other opioid? (Oxycodone/di-acetyl-Morphine 1.5xM, Hydromorphone 6xM, Buprenorphine 50xM + some p-glycoprotein affinity like Loperamide one should note, Fentanyl 100xM, et cetra)

Perhaps even a breakdown of what subtypes it binds to, does it effect Mu-3 subtypes like 6-position esters & ethers of morphine do, for instance?

 

[serotonin2A]

Loperamide binds competitively to opioid receptors amd its affinity can be assessed like any other ligand. You just have to search through the literature to find the Ki values.

 

[Nagelfar]

Loperamide binds competitively to opioid receptors amd its affinity can be assessed like any other ligand. You just have to search through the literature to find the Ki values.

I assumed as much and I have searched the literature, which is why I've had to resort to asking because I have not found it this way. So does anyone have it?

 

[serotonin2A]

I assumed as much and I have searched the literature, which is why I've had to resort to asking because I have not found it this way. So does anyone have it?

I don't want to sound rude but I don't think you put a lot of effort into the search. When I searched for "loperamide mu Ki" on Google, the first entry contained the information you are looking for:

"Loperamide exhibited potent affinity and selectivity for the cloned micro (Ki = 3 nM) compared with the delta (Ki = 48 nM) and kappa (Ki = 1156 nM) human opioid receptors"

http://www.ncbi.nlm.nih.gov/pubmed/10087042

 

[Nagelfar]

So what's that compared to Morphine, as per the usual nomenclature? Would that be roughly x0.5 if morphine = 1.2 nM at Mu or is there another methodology?

 

[sekio]

You'd probably want to read the paper before asking questions like that, they do a direct comparison between loperamide and morphine as well as DAMGO.

You can't really compare Ki values from different binding assays if they use different hot ligands, BTW.

 

[endotropic]

You're interested in the potency of the loperamide preparation relative to morphine, so finding relative ED50's will tell you more than Ki's, since Ki doesn't take into account any pharmacokinetic parameters that effect potency.

 

[serotonin2A]

You're interested in the potency of the loperamide preparation relative to morphine, so finding relative ED50's will tell you more than Ki's, since Ki doesn't take into account any pharmacokinetic parameters that effect potency.

He asked for the "binding potency" so it doesn't seem like he is interested in the ED50.

The ED50 may be listed in the original paper that was cited, because they measured the analgesic activity of loperamide.

 

[endotropic]

He asked for the "binding potency" so it doesn't seem like he is interested in the ED50.

The ED50 may be listed in the original paper that was cited, because they measured the analgesic activity of loperamide.

He asked for a measure of potency and ED50 describes potency, what am I missing here?

They didn't do a full potency curve, but based on the doses they tested ED50 in mouse tail flick looks to be around 1.8mg/kg for loperamide preparation, but they didn't give morphine results to compare with.

edit: This paper used the same antinociceptive assay with the same strain of mice: http://www.ncbi.nlm.nih.gov/pubmed/19729873
ED50 for morphine somewhere around 3mg/kg. The loperamide preparation was hardly more potent than that, and certainly not 10x more potent like the relative affinity would suggest. The coated loperamide probably still loses potency due to p-gp export once the coating dissolves.

 

[serotonin2A]

He asked for a measure of potency and ED50 describes potency, what am I missing here?

They didn't do a full potency curve, but based on the doses they tested ED50 in mouse tail flick looks to be around 1.8mg/kg for loperamide preparation, but they didn't give morphine results to compare with.

edit: This paper used the same antinociceptive assay with the same strain of mice: http://www.ncbi.nlm.nih.gov/pubmed/19729873
ED50 for morphine somewhere around 3mg/kg. The loperamide preparation was hardly more potent than that, and certainly not 10x more potent like the relative affinity would suggest. The coated loperamide probably still loses potency due to p-gp export once the coating dissolves.

I'm just going by his question. He said he specifically wants to know the "binding potency". If that is what he wants info about then knowing the ED50 values that were obtained from a behavioral assay isn't going to answer his question. There are many factors besides binding affinity that influence behavioral activity (functional potency, pharmacokinetics, biotransformation, etc). For example, the mu affinity of fentanyl isn't all that much higher than that of morphine, but it is obviously much more potent in vivo. The situation is similar with loperamide because PK factors influence the in vivo activity of the drug to a greater degree than the receptor binding.

 

[endotropic]

I'm just going by his question. He said he specifically wants to know the "binding potency". If that is what he wants info about then knowing the ED50 values that were obtained from a behavioral assay isn't going to answer his question. There are many factors besides binding affinity that influence behavioral activity (functional potency, pharmacokinetics, biotransformation, etc). For example, the mu affinity of fentanyl isn't all that much higher than that of morphine, but it is obviously much more potent in vivo.

You just described the difference between "binding" and "potency" beautifully. You can't have a "binding potency", binding refers to affinity and potency refers to some functional readout. We already had the binding data covered in this thread and I think the in vivo potency adds valuable information.

The situation is similar with loperamide because PK factors influence the in vivo activity of the drug to a greater degree than the receptor binding.

That's why unless you're involved in drug design or molecular biology measures of potency are almost always more relevant than straight up binding affinity. Obviously potency data from a clinical trial would be even better, but that's clearly not available in this case.

 

[serotonin2A]

You just described the difference between "binding" and "potency" beautifully. You can't have a "binding potency", binding refers to affinity and potency refers to some functional readout. We already had the binding data covered in this thread and I think the in vivo potency adds valuable information.

I know what you are saying and of course I agree with you in general.

Although the question was unclear when it said "binding potency", he asked for the Ki in the title of his post. So I think it is pretty clear that he wants to know the "binding affinity"

That's why unless you're involved in drug design or molecular biology measures of potency are almost always more relevant than straight up binding affinity. Obviously potency data from a clinical trial would be even better, but that's clearly not available in this case.

I don't think that is totally true. Many people here are interested in understanding drug SAR and they specifically want to know about the interactions of the drugs with receptors. My interpretation of this post is that he was thinking that it may be possible to bypass the PK problems with loperamide, but that still leaves the question of how strongly loperamide can interact with the mu receptor. So that is my impression of why he specifically asked for the Ki.

On general, I would agree that ED50 values add nicely to binding data to give an overall sense of a drug's potency. But I don't think that is true in this specific case. There is no reliable way to directly compare the absolute in vivo potency of loperamide and morphine. I guess you could generate a Pgp knockout line, but I don't think they did something like that in the cited study. The study obviously generated an estimate of the potency of loperamide, but is there any reason to believe that their ED50 values reflect the actual potency of loperamide? I suspect the potency estimate would only be relevant to the specific experimental conditions used in their studies, and wouldn't necessatily translate to other situations.

 

[endotropic]

I would expect the relative potency of morphine and this loperamide preparation to translate across different assays (the loperamide just slightly stronger) even though the absolute ED50 numbers would vary. We know what 10mg of morphine does to a patient, and based on everything I've read I'd expect the loperamide to be just slightly stronger, not an order of magnitude stronger like the binding data suggests.

Normally when people compare opioids they're after this sort of equivalency information. All I'm saying is potency readouts get you closer to answering that question than binding data.

I don't think a P-gp knockout mouse would really get at that question, humans have P-gp so our model should to.

 

[Nagelfar]

... measures of potency are almost always more relevant than straight up binding affinity. Obviously potency data from a clinical trial would be even better, but that's clearly not available in this case.

OK, so semantically using the term "binding" in this instance got confused my desire to have a potency relative to morphine given, "behind the theoretical curtain", as it were, of its p-glycoprotein affinity. Too many variables to give? We'd need an actual study of potency with polysorbate 80-coated polybutylcyanoacrylate nanoparticles, is what the case might be?

 

[serotonin2A]

I would expect the relative potency of morphine and this loperamide preparation to translate across different assays (the loperamide just slightly stronger) even though the absolute ED50 numbers would vary. We know what 10mg of morphine does to a patient, and based on everything I've read I'd expect the loperamide to be just slightly stronger, not an order of magnitude stronger like the binding data suggests.

I might not be explaining my thinking clearly enough so here goes...

Lets do a thought experiment where someone tries to compare the ED50s of loperamide and morphine in the tail flick assay in mice by giving the drugs ICV (that way you would theoretically know the exact concentration in the brain). Even in that case, I don't think you could accurately compare the potency of morphine and loperamide. Morphine would have time to diffuse through the ventricular system and would move to its sites of action in brain tissue, but that may not happen with loperamide because it is actively pumped out at a high rate. I'm imagining the brain retaining loperamide as well as a loose sieve holds water. So you would know how much loperamide you had infused, but that may not reflect the actual concentration in the brain.

Normally when people compare opioids they're after this sort of equivalency information. All I'm saying is potency readouts get you closer to answering that question than binding data. I don't think a P-gp knockout mouse would really get at that question, humans have P-gp so our model should to.

True, I can't argue with the use of drug equivalency tables. But I'm thinking that the normal process of calculating drug equivalence may fail in the case of a drug that was developed to have little or no action in the CNS. Obviously loperamide would never be found in such a table under normal conditions.

My general point is that you can compare the behavioral potency of morphine and loperamide, but I can't think of any way to compare them where you would have any confidence that the results accurately reflect their relative functional potency at opiate receptors. I think that PK factors play such a large role in determining the in vivo potency of loperamide that it will be impossible to reach a meaningful conclusion about its receptor effects. The same would be true with opioids such as heroin or tramadol, which produce behavioral effects that are not directly correlated with their binding activity.

 

[endotropic]

OK, so semantically using the term "binding" in this instance got confused my desire to have a potency relative to morphine given, "behind the theoretical curtain", as it were, of its p-glycoprotein affinity. Too many variables to give? We'd need an actual study of potency with polysorbate 80-coated polybutylcyanoacrylate nanoparticles, is what the case might be?

So you want to know, ignoring phamacokinetic variables like distribution to the brain and p-gp export, how potently loperamide activates the opioid receptors compared to morphine? I would find a GTPgammaS assay that looked at un-coated loperamide and morphine in vitro. You wouldn't want to compare the polysorbate 80-coated loperamide in vitro since that coating gets degraded in the body before the free loperamide can bind the receptors, and you couldn't really answer that question in vivo since you can't control for pharmacokinetic differences between the drugs (like serotonin2A pointed out).