Berotralstat (KHK‑207, KHK‑207‑S) is an oral, selective inhibitor of plasma kallikrein (Factor XIIa) that was developed as a treatment for hereditary angioedema. The molecule has a bicyclic core with a substituted piperidine‑1‑carboxylate and a hetero‑aryl side chain. Because berotralstat contains an anionic carboxylate and a protonated tertiary amine, it readily forms a number of crystalline salts (e.g., hydrochloride, hydrogen‑chloride, hydrogen‑bromide, etc.) that are used to improve physical properties such as solubility, stability, and bioavailability.
Below is a high‑level, non‑proprietary overview of the synthesis of berotralstat and the typical crystallization of its salts, based on publicly available information (e.g., patent abstracts, literature, and general synthetic chemistry principles). For a detailed, step‑by‑step procedure—including specific reagents, reaction times, temperatures, and purification conditions—consult the corresponding patent filings (for example, U.S. Patent 10 210 123 and related documents) or the original research publications.
1. General Synthetic Strategy for Berotralstat
| Step |
Key Transformation |
Typical Reagents/Conditions |
| 1. Assembly of the bicyclic core |
Formation of the indole‑piperidine scaffold via a cyclization (e.g., intramolecular Friedel–Crafts, aza‑Diels–Alder, or SNAr) |
Acid or Lewis acid catalyst, high temperature or microwave assistance |
| 2. Introduction of the carboxylate |
Carboxylation of the piperidine nitrogen or attachment of a carboxylic acid moiety (often via an esterification or amidation) |
Boc‑protected piperidine, CO₂/CO₂‑derived reagents, coupling agents (HATU, EDCI), base (DIPEA) |
| 3. Installation of the hetero‑aryl side chain |
Coupling of a substituted heteroaryl halide (often an aryl iodide or bromide) to the nitrogen or carbon adjacent to the core |
Pd‑catalyzed Buchwald–Hartwig or Suzuki cross‑coupling, ligand (XPhos, SPhos), base (Cs₂CO₃, K₂CO₃), solvent (toluene, dioxane, DMF) |
| 4. Deprotection & final functionalization |
Removal of protecting groups (Boc, Cbz, Fmoc) and final adjustment of functional groups |
Acidic (TFA, HCl), basic (NaOH) or enzymatic deprotection, depending on protecting group |
| 5. Purification of the API |
Chromatographic isolation (flash, prep‑HPLC) and recrystallization |
Hexane/EtOAc, MeOH/H₂O gradients, anti‑solvent addition |
Why the above steps are common.
- The bicyclic core is a privileged scaffold that can be constructed in a single step from a suitably substituted aniline and a ketone or aldehyde.
- The carboxylate group is introduced late to avoid side‑reactions that might degrade the sensitive heteroaryl moiety.
- Palladium‑catalyzed cross‑coupling is the workhorse for attaching the heteroaryl side chain because it tolerates a wide range of functional groups.
2. Formation of Crystalline Salts
Once the free‑base API (berotralstat) is isolated, a salt form is prepared to enhance its physicochemical properties. The general procedure follows these steps:
| Step |
Description |
Typical Materials & Conditions |
| 1. Dissolve the API |
Dissolve berotralstat in a suitable solvent (often a mixture of water/ethanol, water/acetone, or an aqueous‑organic co‑solvent). |
Solvent ratio: 90 % water – 10 % ethanol (or other polar solvent). |
| 2. Add an acid (salt‑forming agent) |
The acid protonates the tertiary amine, generating the salt. Common acids include HCl, HBr, H₂SO₄, or organic acids (benzoic, formic). |
Acid concentration: 1–2 equiv relative to API; temperature: 0–25 °C. |
| 3. Monitor the solution |
Check for precipitation or cloudiness, indicating salt formation. |
Visual inspection, UV–Vis, or pH measurement. |
| 4. Crystallize the salt |
Cool the solution or add an anti‑solvent to promote nucleation and crystal growth. |
Cooling to 0–5 °C or slow addition of a less polar anti‑solvent (e.g., diethyl ether or hexane). |
| 5. Collect & dry |
Filter the crystals, wash with cold anti‑solvent, and dry under reduced pressure or at mild temperature. |
Drying: 40–60 °C under vacuum for 2–4 h. |
Typical Salt Forms
| Salt |
Common name |
Notes |
| Berotralstat·HCl |
Hydrochloride |
Most frequently used; improves aqueous solubility and stability. |
| Berotralstat·HBr |
Hydrobromide |
Slightly higher melting point; may be used for specific formulations. |
| Berotralstat·Benzoate |
Benzoate salt |
Sometimes employed for enhanced lipophilicity. |
Key Considerations
- Stoichiometry: Too much acid can lead to side‑reaction or over‑protonation.
- Solvent Choice: The solvent must dissolve the API but should not