does anyone know why this particular drug has such a different effect profile, depending on the ROA?
Speed of absorption and metabolic factors. If a substance is converted to an inactive one or to one that has a different effects profile, the conversion will happen at a particular rate dependent on the plasma concentration, genetic factors, state of your liver, kidneys, other medication etc.
Generally speaking, faster absorption will maximize the effects.
Not only will there be a higher peak plasma concentration, simply because the body usually can't metabolize a substance as fast as it is being absorbed, but the reaction of your cells will usually also be stronger. This is due to the cellular adaptive mechanisms that take place upon activation of a target cell.
There are usually negative feedback mechanisms involved when it comes to monitoring the activation of receptors. You flood your receptors with a substance, the target receptor proteins will cause a reaction, this reaction is registered and measures are taken to decrease the impact like downregulation. A cell can for example move transmembranous receptor proteins to the inside of the cell in order to desensitize itself towards the incoming ligands. This is part of why a tolerance to most drugs develops.
Other possibilities are that a factor important to the metabolization is being produced or released to a greater extent, e.g. many substances induce or inhibit production of enzyme proteins from the CYP P450 family, but this mechanism is not limited to them. Maybe some coenzymes that were kept back before will now be released and make processing of the substance easier for enzymes that are already produced.
These feedback mechanisms are often not limited to the targeted cells alone, but possibly to other tissue in your body as well.
So when a substance is absorbed very quickly, the body doesn't have sufficient time to
1) adapt in order to desensitize
2) metabolize or excrete the substance
A great example for quick desensitization is MDMA which causes MUCH stronger reactions when taken rectally or when it is injected intravenously. It releases MDMA from the serotonergic presynapse which then floods the postsynaptic serotonin receptors. This desensitization NOT only happens at the target cell, but also in the cell that releases the substance (into which it is transported back by other transmembranous proteins).
An example for quick metabolization is heroin (desensitization also plays a big role though). It's metabolized so quickly that before a full oral dose has been absorbed most of it has already been gotten rid of. Therefore the instant flooding via inhalation or intravenous injection means that when a person has finished his shot/smoke, most of the heroin is still active.
Most arylcyclohexylamines undergo extensive metabolization by a large number of different proteins, resulting in a plethora or different metabolites. One of ketamine's metabolites for example is responsible for it's antidepressant action, despite not causing you to trip.
It is not clear how exactly 2-Oxo-PCE is metabolized afaik, but the usual suspects (CYP P450) are known to do catalyze small number of chemical reactions better than other reactions. E.g. one is very good at N-demethylation of monoamines, another O-demethylates or deacetylate. You can therefore make assumptions that protein X catalyzes process Y at a position Z of substance XYZ. I think you get the idea. Most RC's are never really investigated for what is their fate inside the body. What is relatively common is just throwing them in a dish with various enzymes and look at the results or deduce from the metabolites found in the blood, urine and stool.
One more thing is distribution throughout your body. The substance might be present in more or less equal amounts across all tissues after entering the bloodstream, but are taken up better by some tissue over other tissue and therefore the activity in the different organs can start varying greatly after that. In some cases this is due to the body's intention to metabolize them or simply keep them out of other tissue (think glia cells that contribute to the BBB), in other instances the physical properties of a substance dictate where they end up, simply because different tissue has different properties. E.g. some substance dissolve better in more polar environments than others (polarity or logP O/W).
Lol I was only gonna write two sentences.