Receptor binding and drug dose

[J Hicks]

Hi all,

I have a question about how drug dose relates to receptor binding efficiency.

I have attached a photo of some peptides designed as novel MC1R receptor agonists from the following paper:

https://www.researchgate.net/publication/320823039_Design_of_MC1R_Selective_g-MSH_Analogues_with_Canonical_Amino_Acids_Leads_to_Potency_and_Pigmentation

The question is this:

Substance 5 on the list shows an EC(50) on the MC1R receptor of 4.5nM, whilst substance 15 shows an EC(50) of 0.01nM.

Does this effectively mean that substance 5 requires a 450 fold dose to match the potential effect of substance 15, assuming that pharmacokinetics are roughly equal?

Any thoughts you might have would be welcome.

 

Cheers guys.

[BabcockHall]

What are your thoughts?  Once we know those, then we may be able to help you.

[J Hicks]

On 03/03/2018 at 4:36 PM, BabcockHall said:

What are your thoughts?  Once we know those, then we may be able to help you.

We'll I'm wondering if substance 5 would require a much higher dose in vivo than substance 15 to activate the mc1r receptor. 

[BabcockHall]

Is this a homework problem?

[J Hicks]

No, it's not a homework problem.

[J Hicks]

Can anyone recommend a forum where you might actually get an answer to a question like this? As opposed to just being interrogated about your reason for posting and then ignored?

Edited March 16, 2018 by J Hicks

[StringJunky]

2 minutes ago, J Hicks said:

Can anyone recommend a forum where you might actually get an answer to a question like this? As opposed to just being interrogated about your reason for posting and then ignored?

People want to know so that they can respond appropriately.

[CharonY]

Think about how EC50 is calculated and what it represents. To reach the same half maximum occupancy the values can be directly compared. But what for other levels?

[BabcockHall]

On 3/6/2018 at 2:52 PM, J Hicks said:

We'll I'm wondering if substance 5 would require a much higher dose in vivo than substance 15 to activate the mc1r receptor. 

That's just a restatement of what you said in your opening post.  I expected more effort than you have put in so far, such as defining EC_50, or saying that EC₅₀ was measured by examining cAMP levels.  I had to go to the original paper to learn that.

Edited March 22, 2018 by BabcockHall

[J Hicks]

4 hours ago, BabcockHall said:

That's just a restatement of what you said in your opening post.  I expected more effort than you have put in so far, such as defining EC_50, or saying that EC₅₀ was measured by examining cAMP levels.  I had to go to the original paper to learn that.

In other words, you don't know the answer, so why not reserve the pointless, condescending comments? For those who actually understand the question, substance 15 is dosed at 1mg per day to stimulate mc1r receptors, I'm looking to understand if substance 5 is likely to stimulate the same receptor at doses of around 5-10mg, rather than 500mg, as implied by the EC(50).

 

Anyone with knowledge of peptide pharmacokinetics or receptor agonists have an opinion?

[BabcockHall]

First, we are not here to do your homework or your non-homework thinking for you.  You are supposed to show some sort of effort.  Second, you did a poor job presenting the question.  Table 2 is cut off in your diagram.  Third, why did you ask for someone with a knowledge of peptide pharmacokinetics?  Pharmacokinetics could be defined as "the study of the time course of drug absorption, distribution, metabolism, and excretion".  The question you are asking is not entirely unrelated to that field, but we don't have any information that bears on the pharmacodynamics of either compound (and in any case your opening post assumes identical pharmacokinetics).   Fourth, the answer to your opening question could be found by thinking carefully about the reply you have already been given by CharonY.  Your most recent answer suggests to me that you are not approaching this problem correctly.

Consider the following hypothetical.  Compound A has a value of EC₅₀ of 1 nM.  Compound B has a value of EC₅₀ has a EC₅₀ value of 10 nM.  Suppose that both are present in vivo at 100 nM.  What would you conclude?

Edited March 22, 2018 by BabcockHall

[hypervalent_iodine]

1 hour ago, J Hicks said:

 I'm looking to understand if substance 5 is likely to stimulate the same receptor at doses of around 5-10mg, rather than 500mg, as implied by the EC(50).

 

And you were prompted to think about what the EC₅₀ means, and what it might imply in terms of your question. Perhaps think about that and share your thoughts rather than constantly restating the question. We're happy to discuss, but you're not giving us a lot to work with. 

[BabcockHall]

18 hours ago, J Hicks said:

...substance 15 is dosed at 1mg per day to stimulate mc1r receptors, I'm looking to understand if substance 5 is likely to stimulate the same receptor at doses of around 5-10mg, rather than 500mg, as implied by the EC(50).

This part of your comment provides some context.  In order to answer the question, we may need to make the assumption that the concentrations in vivo are proportional to the dose.

Let Y be the response and X be the concentration of agonist.  One equation that can be used is Y = [(a - d)/(1 + (X/c)^b)] + d.  The parameter a is lower asymptote or plateau; b is the slope factor; c is the concentration of agonist that provokes a response halfway between the baseline response and the maximum response; and d is the upper asymptote or plateau.  The authors of the J. Med. Chem. paper may cited previous papers; therefore, I am not certain whether or not they used this equation.  How does parameter c relate to your question?

Edited March 23, 2018 by BabcockHall

[BabcockHall]

I recommend reading Motulsky and Christopoulos, p. 256 and p. 315.  It is often the case that the response Y is graphed versus [agonist], where the latter is plotted on a logarithmic scale.  If one wishes to set the Hill slope to unity, then one can fit to a three-parameter equation.  Y = bottom + (top - bottom)/(1 + 10^log(EC50 - X)).  If one wants to include the Hill slope as the fourth parameter, then the term (EC50 - X) should be multiplied by the Hill slope.The shape of this curve is sigmoidal, and its steepness is governed by the Hill slope.

Edited March 24, 2018 by BabcockHall

[BabcockHall]

@OP, What does the concept of saturation mean to you?

[J Hicks]

Many thanks for your help with this.

So, from what I understand here, if compound 5 reaches a molarity of 4.5nM or higher in the plasma, it will thus achieve the EC(50) level of agonist activity for the MC1 receptor.

So the pertinent information/data here would be the dose and related CMAX from a pharmacokinetic profile of the substance. However, as there are no studies at this time on the pharmacokinetics of the compound, any answers will only be theoretical unfortunately.

[BabcockHall]

I would say that when compound 5 reaches 4.5 nM, it reaches its EC50, the midpoint between the effect at zero concentration, and the effect at saturation.  If its concentration is, say 20-fold, above 4.5 nM (90 nM), then it is near saturation, and any further increase would have little effect.  Qualitatively, the same holds true for compound 15, except that its EC50 means that it saturates at a much lower concentration.  I am not sure what you mean by CMAX; however, one column in the table that you provided suggested to me that both compounds produce the same maximal effect at high concentration.   I think that we would need to know the in vivo concentrations of 5 and 15 to be certain.  Yet, if both are high relative to their EC50s, then I suspect that they will produce similar effects. 

https://www.graphpad.com/guides/prism/7/curve-fitting/reg_the_ec50.htm?toc=0&printWindow

Edited March 27, 2018 by BabcockHall

[J Hicks]

CMAX refers to the maximum concentration of a specific compound in the plasma following a specific dose.

[BabcockHall]

In this instance, I would take it that the compound is cAMP and that the column of data marked %max dose refers to how much cAMP is generated by a high concentration of the peptide, measured using a relative scale.  Both 5 and 15 scored 100%. MT-II is only mentioned twice in the paper, and I am still working out what it means.

[CharonY]

I do feel that at least with respect to the question the discussion is going a bit astride. Again, EC50 is a summary point for dose-response curves. Knowing baseline and max response are necessary to define this point. However, it does not tell you much about the shape of the curve, which you would need if you want to compare arbitrary doses outside of the EC50. The max response is relevant if we talk about efficacy (as opposed to potency).

[J Hicks]

MT-II is another peptide analogue of MSH with high binding affinity to the MC1R receptor. Thus it is being used as a control in this study. Is it possible to figure the dose response curve of substance 5 from the information laid out in the paper? From what is being said here in this thread, a dose response curve would illustrate the changing effects of substance 5, rather than merely the EC(50).

Also, I assume the pharmacokinetic profile of peptides varies by the peptide type, so there is no way to predict the likely cMAX from substance 5, without an actual trial being done.

It seems to me that these two pieces of information, the dose response curve, and the cMAX are the key pieces of information required to understand the drug dose required to stimulate the MC1R receptor.

Edited April 1, 2018 by J Hicks

[BabcockHall]

I think we would need more parameters, such as the two asymptotes.  Possibly earlier papers in the series provides some information.