**4. Conclusions**

In conclusion, we have described two new drug–drug cocrystals containing FUR and ETZ and PRX as coformers. A mechanochemical synthetic route have allowed to avoid hydrate/solvate formation as evidenced in the case of FUR-PRX. Expected hydrogen bonds contributed by the drug coformers sustain the cocrystals, disrupting the acid:aciddimer synthon observed in the stable FUR polymorph 1. All the solids exhibit good thermal stability, and good stability under accelerated ageing. Although they do not exhibit increased solubility than FUR drug, the drug coformers notably do. However, the success of these drug–drug cocrystals as potential fixed-dose solids requires an appropriately designed clinical study to establish their safety and effectiveness.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/article/10.339 0/cryst11111339/s1, Figure S1. PXRD patterns of FUR–ETZ obtained by grinding the two components at different molar ratios. Blue dotted lines indicate characteristic FUR reflections. Orange dotted lines indicate characteristic ETZ reflections; Figure S2. PXRD patterns of FUR–PRX obtained by grinding the two components at different molar ratios. Blue dotted lines indicate characteristic FUR reflections. Orange dotted lines indicate characteristic PRX reflections; Figure S3. Le bail profile fit (red line) to the experimental PXRD data (blue line) of FUR-ETZ (a) and FUR-PRX (b). The profile fitting for both the cocrystals shows low discrepancy (grey line); Figure S4. ORTEP representation showing the asymmetric unit of FUR—ETZ with atom numbering scheme (thermal ellipsoids are plotted with the 50% probability level); Figure S5. ORTEP representation showing the asymmetric unit of FUR—PRX with atom numbering scheme (thermal ellipsoids are plotted with the 50% probability level); Figure S6. TGA traces of FUR–ETZ (top) and FUR–PRX (bottom); Figure S7. Solubility curve of FUR—ETZ in water PBS at pH 7.4; Figure S8. Solubility curve of FUR—PRX in water PBS at pH 7.4; Figure S9. PXRD patterns of FUR–ETZ after the stability slurry assay (at 25 ◦C, during 24 h, in water); Figure S10. PXRD patterns of FUR–PRX after the stability slurry assay (at 25 ◦C, during 24 h, in water); Table S1. Hydrogen bonds for FUR—ETZ (Å and deg.); Table S2. π,π-stacking interactions analysis of compound FUR—ETZ. Table S3. Hydrogen bonds for FUR—PRX (Å and deg.).

**Author Contributions:** Conceptualization and methodology, D.C.-L.; Formal analysis and investigation, C.A.-P., L.R.-D., F.J.A.-M., A.D.-M. and J.G.-M.; Writing—original draft preparation, D.C.-L.; Writing—review and editing, D.C.-L.; Funding acquisition, D.C.-L. and J.G.-M.; Supervision, D.C.-L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Spanish Agencia Estatal de Investigación of the Ministerio de Ciencia, Innovación y Universidades (MICIU) and co-funded with FEDER, UE, Project No. PGC2018- 102047-B-I00 (MCIU/AEI/FEDER, UE) and Project No. B-FQM-478-UGR20 (FEDER-Universidad de Granada, Spain).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** F.J.A.-M. wants to acknowledge an FPI grant (ref. PRE2019-088832).

**Conflicts of Interest:** The authors declare no conflict of interest.
