Some questions on halogenated amphetamines

[aced126]

So I read this study by Mr. Fuller (co-discoverer of Prozac) on SAR of halogenated amphetamines and related neurotoxicity.

http://bitnest.ca/Silo42/10.1111/j.1749-6632.1978.tb31518.x.pdf

In the paper they mention a couple of things which raises questions in my head.

1) The findings suggest that after one dose of pCA administered i.v. 5HT and 5HIAA concentrations were reduced and stayed reduced for up to 4 months in rats. One of the likely reasons of this was that TPH was inhibited. It does also indicate that oCA and mCA did not have this effect, but after pre-treatment with drugs that block para-hydroxylation, oCA and mCA reduced 5HT concentrations in the same way that pCA did. Now what I'm thinking is that this suggests pCA remains in the brain for a longer time. But it definitely can't stay there for 4 months. Surely beta hydroxylation or other metabolic pathways will proceed eventually to excrete the drug. Does this mean pCA (and oCA and mCA when p-hydroxylation is inhibited) irreversibly binds to and inactivates TPH? Or does inhibition occur through another mechanism? If pCA does irreversibly bind, then why wouldn't oCA and mCA? Obviously another explanation could be that the neurons itself have been destroyed, but this could be challenged by administering 5HTP, circumventing the need for TPH itself, and observing the concentration of 5HT afterwards.

2) In guinea pigs, mCA had the same effect as pCA because para-hydroxylation did not occur. It seems para-hydroxylation is key to preventing neurotoxicity because the drug can then be excreted quickly, in comparison to other metabolic pathways like beta-hydroxylation. Why is this so? P-hydroxylated amphetamines can't be that much more polar than products of other metabolic pathways? Nevertheless, in guinea pigs oCA actually caused an increase in 5HT concentration over time, which confuses me a lot. o,p-dichloroamphetamine had a significantly less reduction on 5HT than pCA. Why does ortho substitution increase 5HT concentration but para and meta decrease it?

3) Administration of fluoxetine shortly after (4 hours) administration of pCA or pBA reverses the reduction. However, the degree of reversal reduces over time. This suggests these compounds are taken up into the neuron by SERT. Why can't they simply diffuse through the membrane like amphetamine into dopaminergic neurons?

4) B,B-difluoro-pCA has a lower pKa than pCA (6.8 and 9.4 respectively). This results in the former compound remaining in neutral form. I thought this would enable more efficient crossing of the BBB, but instead the compound accumulates in adipose tissue, and one needs to administer 4-5 times the molar amount to achieve similar concentrations in the brain. If this is so then surely the brain concentration of highly lipophilic drugs can be increased if the molecule was made polar?

 

[serotonin2A]

1) The Cl-amphetamines do not inhibit TH directly (they even say that in the article). The Cl-amphetamines either denature it due to oxidative stress and/or because degenerating neurons stop making TH.

3) Very little amphetamine gets into dopamine neurons by passive diffusion. The rate at which amph diffiffuses into DA neurons is much lower then the amount that is pumped in by the transporter.

 

[aced126]

Thanks for your response. A few more questions...

In table 3, it shows that after administration 5HT concentrations start to decrease immediately. Where is this 5HT going anyway then? 5HIAA concentrations get reduced as well, suggesting the 5HT isn't getting metabolised. If this is so, how does 4CA have antidepressant action. Furthermore, how does 4FA have MDMA-like action? At least initially one would expect increased extracellular 5HT concentration. Once the extracellular 5HT is metabolised, then one could expect decreased 5HT concentration both inside the neuron and in the synaptic cleft due to TPH inhibition and other mechanisms. Why does 5HT concentration go down immediately?

 

[Dresden]

I don't know, but if all alkyloxy-amphetamines were judged based on PMA as the paradigm (as PCA is of the halo-amphetamines), we'd be in the same place we're in with them as we are with the halo-amphetamines: Nowhere. And techno music and all-night raving as we know it never would have proliferated in the U.S. like it did for a while (before sassafras oil was so tightly controlled).

 

[serotonin2A]

Thanks for your response. A few more questions...

In table 3, it shows that after administration 5HT concentrations start to decrease immediately. Where is this 5HT going anyway then? 5HIAA concentrations get reduced as well, suggesting the 5HT isn't getting metabolised. If this is so, how does 4CA have antidepressant action. Furthermore, how does 4FA have MDMA-like action? At least initially one would expect increased extracellular 5HT concentration. Once the extracellular 5HT is metabolised, then one could expect decreased 5HT concentration both inside the neuron and in the synaptic cleft due to TPH inhibition and other mechanisms. Why does 5HT concentration go down immediately?

The fluorometric detection method probably isn't sensitive enough to measure extracellular serotonin. It is designed to detect serotonin at the relatively high concentrations found in cytoplasm and vesicles.

The reduction they detected is probably due to release of serotonin.

Another possible explanation is that some of the serotonin is metabolized to substances other than 5-HIAA.

 

[adder]

4) B,B-difluoro-pCA has a lower pKa than pCA (6.8 and 9.4 respectively). This results in the former compound remaining in neutral form. I thought this would enable more efficient crossing of the BBB, but instead the compound accumulates in adipose tissue, and one needs to administer 4-5 times the molar amount to achieve similar concentrations in the brain. If this is so then surely the brain concentration of highly lipophilic drugs can be increased if the molecule was made polar?

As for this one, I guess due to lower basicity of difluoro-pCA it is less potent because the interaction with SERT is likely largely governed by proton transfer/hydrogen bonding, so the extent of protonation of the amine is important (there must be an aminoacid residue with negatively charged side-chain).

From what I noticed compounds that cross the blood-brain barrier very fast and effectively without much of it getting stuck in fat tissues (they may accumulate anyway after leaving the brain though) are lipophilic compounds with polar regions. Lipophilic compounds without polar sites will usually cross the BBB very slowly and once they're in the brain, they are cleared very slowly as well, e.g. PCP. Another example is levorphanol, compared to morphine or heroin it is fairly slowly acting (by i.v. route the come-up is gradual, there is no instant rush), but if you only add the ether bridge as in desomorphine, suddenly the compound is very fast-acting. So as long as a compound doesn't get too hydrophilic, it seems to benefit from more polarity. I guess crossing the BBB has a lot to do with the number of polar regions which allow partial charges to move through a molecule more effectively (bond polarization).

 

[aced126]

What effects would highly electron donating groups on the beta carbon have other than steric effects?

 

[adder]

Well, I don't think the effect would be so obvious, most groups that are electron-donating when on the aromatic ring have both electron-donating and electron-withdrawing properties through resonance and induction, and resonance effect most often dominates on the aromatic ring. In case of beta,beta-difluoro-amphetamine the effect of the electron-withdrawing fluorine atoms may be quite complex too, but nevertheless due to high electronegativity of fluorine much more uniform (i.e. withdrawing). The fluorine atoms are likely to interact with hydrogen atoms on the amine (and hydrogen atoms of an alkyl group on the amine if it's secondary or tertiary) and thus interact with the amine more directly, and they certainly also withdraw electron density from the aromatic ring (compare Ar-CF3 and Ar-CF2-R). With a group like hydroxy the inductive effect dominates if it's attached to an aliphatic chain and unless you have some multiple bonds, the EDG group can't have that much effect on groups a few carbon atoms away. Compare pKa values for ephedrine/norephedrine and methamphetamine/amphetamine, they're very similar and amphetamine counterparts are actually a tiny bit more basic which could be due to -I effect of the beta-hydroxy group (substituents on the aromatic ring will also have some slight effect + perhaps an even less pronounced effect due to different sterics in different isomers).

link 1
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[εὔνοια]

Sorry for digging up an old post. Your questions in this post are interesting. Just a couple questions/thoughts:
- Did you ever figure out why 5HT and 5HIAA concentrations went down in pCA administered rats, but not in oCA or mCA (unless pre-treated to block para-hydroxylation)? It looks to me like a scenario in which a low-affinity, endogenous ligand usually occupies a specific location/configuration on the pCA-receptor. Perhaps the presence of this endogenous ligand plays a second messenger role that is vital in the expression of TPH.

 

[εὔνοια]

Sorry for digging up an old post. Your questions in this post are interesting. Just a couple questions/thoughts:
1)
- Did you ever figure out why 5HT and 5HIAA concentrations went down in pCA administered rats, but not in oCA or mCA (unless pre-treated to block para-hydroxylation)? It looks to me like a scenario in which a low-affinity, endogenous ligand usually occupies a specific location/configuration on the pCA-receptor. Perhaps the presence of this endogenous ligand plays a second messenger role that is vital in the expression of TPH. This would explain the seeming anomaly of oCA and mCA producing uncharacteristic effects in the presence of a para-hydroxylation blocker. It would also explain why the effect is pronounced in rats given pCA so long as you can classify pCA as a drug with much higher affinity for the pCA-receptor than the proposed endogenous ligand.

- The results showing reduced 5HT and 5HIAA levels for 4 months, in my mind, pretty much leaves one option. Neuronal cell death in a particular cell population. Even in the event of the strongest binding possible (covalent), the receptor would simply become internalized and it would occur WAY faster than 4 months. Now maybe I'm wrong and it just takes that long for the receptor environment to normalize, but I suspect that is no the case here. My only other guess is that drugs like 4CA act as selective seratonin reuptake enhancers - which have surprisingly enough been shown to produce antidepressant effects.a

2)
- It starts getting tricky when talking about a system that may not have been conserved in a similar way. Even still though, the behavior behind different p-o-r configurations even across species suggests that there is some kind of secondary binding site or endogenous ligand.

- As far as to why P-hydroxylated amphetamines display this, I don't know. But it can always have to do with things other than just sheer polarity. Steric configuration might give the molecule an increased capacity to stimulate different players in the neurotoxicity cascade.

3)
- I feel like the activity of SERT on pCA and pBA is a big hint at a mechanism that we are not yet aware of. These para-hydroxylated amphetamines are being treated very strangely and appear to have a very strong interaction with multiple components of the seratonergic system in the Raphe Nucleus.

- As for fluoxetine ameliorating the effects, that is expected and gets us nowhere in particular as far as the mechanism behind all of these phenomenon. I only say this because fluoxetine is simply going to reduce the reuptake of 5HT which would obviously increase synaptic levels. Maybe some useful info could be gleaned by observing any variations among SSRI impacts on 5HT levels (post treatment with pCA or pBA of course).

Anyways, good post. Thanks.

 

[serotonin2A]

You are overlooking much of what is known about the action of pCA. pCA is taken up by SERT into serotonergic neurons, where it causes reverse transport of serotonin. The uptake of pCA also causes some of the cells to degenerate, although the mechanism is unclear. pCA is the most potent of the Cl-amphetamine isomers, explaining why serotonin depletion shows the relationship: pCA > oCA/mCA. The potency is determined by the affinity of the isomers for SERT.

The results showing reduced 5HT and 5HIAA levels for 4 months, in my mind, pretty much leaves one option. Neuronal cell death in a particular cell population. Even in the event of the strongest binding possible (covalent), the receptor would simply become internalized and it would occur WAY faster than 4 months. Now maybe I'm wrong and it just takes that long for the receptor environment to normalize, but I suspect that is no the case here.

It is well established that pCA causes serotonergic neurons to degenerate. Hence why 5-HT, 5-HIAA, and TPH levels are reduced for several months.

2)
- It starts getting tricky when talking about a system that may not have been conserved in a similar way. Even still though, the behavior behind different p-o-r configurations even across species suggests that there is some kind of secondary binding site or endogenous ligand.

I don't understand exactly what you are proposing?

3)
- I feel like the activity of SERT on pCA and pBA is a big hint at a mechanism that we are not yet aware of.

pCA is transported into serotonergic neurons by SERT. That is why fluoxetine and other SSRIs block its effects on serotonin release and neurotoxicity.