Also my opinion on this is
Yep, anhydrous raecemic MDMA hydrochloride does indeed form three identified polymorphs.
Also at least four hydrates.
Form III spontaneously converts to form I, Form II will convert to Form I under competitive equilibration conditions. In aqueous conditions, hydration occurs. As they point out, the well known Form I is the most stable.
It does seem to suggest all are readily soluble in water. Or, rather, all revert to Form I and then undergo hydration.
So I'm uncertain if this makes a blind bit of difference in the case of MDMA hydrochloride.
But it IS a viable way of obtaining a patent. I stand by that reading; that it's to get patent protection rather than having any real world advantage(s). Might be physically smaller, if that IS an advantage?
Different addition salts certainly DID alter the pharmokinetics of a related compound, but inherently changing the addition salt means the crystalline structure(s) will be different anyway. Maybe that's one reason why MDMA.HBr turned up on occassion? It's impossible to know why in most cases, but we just happened to get complaints so actually had to look into it.
If nothing else, the paper does demonstrate two less soluble impurities, 2-chloro and 2-bromo MDMA. Even a tiny amount of an impurity has the porential to wreck havok on crystalization.
I can only GUESS that 'moonrock' MDMA MAY be in an one of the less stable forms if seeding used a crystal of Form II or Form III, but in the presence of impurities? I don't know. Smaller WOULD be an advantage if it is to be smuggled.
I am aware of two cases where it mattered. The Ritonavir production-line suddenly began producing a much less soluble second form and manufacture was abandoned. The other was Rotigotine patches in which spontaneous polymorphic transition resulted in a less soluble form so a less active medicine. I don't say other examples simply don't exist, but those were the two that ended up in the news.
But I can follow the thinking...
Fast melt slow recrystallization does a few things smuggling space, if one pot or telescopic synthesis is done, it will be a good way to force a lot but not all impurities, Impurities also tend to be missed under basic spectre, speed of productionetc etc etc.
The real issue though is fast melt in an API setting is by in large not traditionally done.
Next when it came to ritonavir the most interesting thing is polymorph#3/ #4 madevia melt
Nobody knew about polymorph #3/4 until 2023/2024/ 2025/2026
Because fast melt can create more unknown polymorphs. And because maps to my knowledge has not fast melt recrystallization it does by in large add up
Two new polymorphs of piroxicam (Forms VI and VII) were discovered by melt crystallization and crystal structure prediction (CSP).
Surprisingly, even though melt crystallization has a long history, it has been employed less often in the search for new polymorphs than solution crystallization. Applications of melt crystallization to 21 highly polymorphic, well-characterized compounds with at least five ambient polymorphs revealed that melt crystallization afforded more than half of the known polymorphs and in many cases revealed new polymorphs not detected by other screening methods. A statistical analysis revealed that polymorphs grown from the melt have a greater propensity for high Z′ values, which are not easily accessible by other crystallization protocols and are often not detectable by crystal structure prediction methods. Melt crystallization within nanopores (8–100 nm) performed for 19 of the 21 compounds mostly resulted in polymorphs that dominated crystallization from the bulk melt at similar temperatures. The total number of polymorphs observed in nanopores was less than that observed during crystallization from the bulk melt, however, and melt crystallization under confinement revealed new polymorphs not detected by other crystallization methods.
Nonetheless, 24 years after the appearance of Form II of Ritonavir, scientists at AbbVie Inc. serendipitously discovered a new polymorph while studying the crystal nucleation of amorphous Ritonavir, and obtained the new Form III via melt crystallization. [9]
26-27 years 2 years after #3 form IV was found
The hydrogen-bonding network in Form IV differs from other forms, often characterized by alcohol-amide interactions. form 4 contains a trans carbamate configuration, like unstable forms 1 and 3, and unlike stable form 2. The hydrogen bonding network of form 4 is shown in Fig. 2. Consistent with the conformational similarity with forms 1 and 3, it has the amides forming a continuous hydrogen-bonded chain. However, there are also some notable differences. In form 1, the alcohol forms an additional intermolecular hydrogen bond with the nitrogen atom of the thiazole. In contrast, in form 4, the hydrogen bond is formed with one of the amide oxygen atoms. Meanwhile, form 3 displays a mix of the two: two of the four molecules in the asymmetric unit create the alcohol-amide hydrogen bond seen in form 4, while the other two form the alcohol-thiazole bond found in form 1. From this analysis, it appears that the more complete hydrogen bonding pattern contained in form 2, which is responsible for its stability, can only be obtained compromising the conformational energy and introducing the unstable cis carbamate configuration1
So, how many solid forms are still unknown?
Anyone who has tried to crystallise a stubborn compound will know that the mysterious business of coaxing molecules out of solution and into a regular solid array is very much a dark art. Not everything can be crystallised, and the structure you get if you do produce diffractable crystals is almost as much of a mystery. It’s still very hard to predict how organic molecules will pack in three dimensions, as James Mitchell Crow’s feature on crystal structure prediction explains.
Ritonavir form 4 could not be solved by the “gold standard” of single-crystal XRD, but a combination of 3D-ED with computational methods and experimental XRPD data produced an accurate solution and showed it is significantly less stable than forms 1 and 249. Being able to solve crystal structures for which single-crystal XRD is not an option is important from a risk assessment perspective. Often, high throughput experimentation finds new crystalline forms that are just seen as peaks on XRPD patterns, without the possibility of isolating them, solving their structures, and verifying their relative stabilities. This can cause significant concerns in drug development. Ritonavir form 4 ended up being unstable, but there are cases in which solving a crystal structure from a small sample size and therefore determining its relative thermodynamic stability is fundamental to quantify and assess risks. In this scenario, leveraging less traditional methods like 3D-ED or a combination of CSP and experimental data51–53 is the only option. This ability to characterize crystal structures from a small sample size can also have intellectual property implications54.
Finally, the work reveals the importance of determining structural disorder in crystal structures and quantifying its effects on the stability of polymorphs for a complete risk assessment on the polymorphic landscape of drug candidates. Disorder has a modest but not irrelevant stabilizing effect on ritonavir form 1 and particularly on form 4, and likely on form 3 too (where the effect was not computed), relative to ordered form 2. Crystalline disorder and its stabilizing effect are getting increasingly recognized in the solid-state community, and this study shows that this interest is well deserved.
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