Pemirolast (INN) is a mast cell stabilizer used as an anti-allergic drug therapy. It is marketed under the tradenames Alegysal and Alamast.

9-methyl-3-(1H-tetrazol-5-yl)-4H-pyrido-[1, 2-a]-pyrimidin-4-one

It has also been studied for the treatment of asthma.

https://www.google.com/patents/US9006431

Pemirolast is an orally-active anti-allergic drug which is used in the treatment of conditions such as asthma, allergic rhinitis and conjunctivitis. See, for example, U.S. Pat. No. 4,122,274, European Patent Applications EP 316 174 and EP 1 285 921, Yanagihara et al, Japanese Journal of Pharmacology, 51, 93 (1989) and Drugs of Today, 28, 29 (1992). The drug is presently marketed in e.g. Japan as the potassium salt under the trademark ALEGYSAL™.

Commercial pemirolast potassium has the disadvantage that it is known to give rise to sharp plasma concentration peaks in humans (see, for example, Kinbara et al, “Plasma Level and Urinary Excretion of TBX in Humans”, Japanese Pharmacology & Therapeutics, 18(3) (1990), and “Antiallergic agent—ALEGYSAL tablet 5 mg—ALEGYSAL tablet 10 mg—ALEGYSAL dry syrup”, Pharmaceutical Interview Form (IF), Revised in October 2007 (7th version), Standard Commodity Classification No.: 87449). The latter document also reports that the potassium salt of pemirolast is hygroscopic, which is believed to give rise to chemical instability, and possesses a bitter taste.

U.S. Pat. No. 4,122,274 describes a process for the production of salts of pemirolast, including potassium salts and (at Example 14) a sodium salt. As described herein, this technique produces a sodium salt that is physically unstable. Sodium salts of pemirolast are also mentioned (but a synthesis thereof not described) in international patent applications WO 2008/074975 and WO 2008/075028.

COMPARATIVE EXAMPLE 5Recrystallisation of Pemirolast Sodium According to the Method of U.S. Pat. No. 4,122,274

In U.S. Pat. No. 4,122,274, it is stated that the crude title product (pemirolast sodium) was recrystallised from water:ethanol to give pure title product. It is not clear from this level of detail what the ratio of water:ethanol employed was, so several experiments were performed with a view to reproducing the prior art technique.

• (i) Crude sodium salt of pemirolast (480 mg; from Example 4, method (I) above) was recrystallised from water and ethanol (95%) in a 1:1 ratio. The Na salt of pemirolast (480 mg, 1.92 mmol) was dissolved in H₂O (8 mL) at 70° C. and EtOH 95% (8 mL) was added. The clear solution was allowed to reach room temperature and the solid material formed was filtered off, washed with a small amount of ethanol and dried in vacuum to give 316 mg of pure sodium salt.
• (ii) Crude sodium salt of pemirolast (500 mg; from Example 4, method (II) above) was dissolved in water (4.9 mL) at 70° C. Thereafter EtOH 95% (ca. 4.0 mL) was added at 70° C. until a solid started to form. Another 0.1 mL of water was added to get everything into solution. The solid material formed upon cooling was collected by filtration and dried under vacuum to give 348 mg of pure sodium salt.
• (iii) Crude sodium salt of pemirolast (300 mg; from Example 4, method (II) above) was recrystallised from water:ethanol (1:1 ratio; 10 mL) at 70° C. The solid material formed upon cooling was collected by filtration and dried under vacuum to give 174 mg of pure sodium salt.
• (iv) Crude sodium salt of pemirolast (300 mg; from Example 4, method (II) above) was recrystallised from water:ethanol (9:1 ratio, 4 mL) at 70° C. The solid material formed upon cooling was collected by filtration and dried under vacuum to give 219 mg of pure sodium salt.

All four samples of pure pemirolast sodium salt had the same physico-chemical properties (Raman spectra and NMR):

¹H NMR (D₂O) δ: 8.86-8.80 (m, 1H, CH), 8.57 (s, 1H, CH), 7.68-7.59 (m, 1H, CH), 7.22-7.13 (m, 1H, CH), 2.39 (s, 3H, CH₃).

The PXRD pattern (measured in respect of Example 5(i) above) is shown in FIG. 3. It was concluded from this that this form of the sodium salt is an amorphous material mixed with a crystalline fraction.

The Raman spectrum was recorded directly after recrystallisation. All samples were then stored under ambient conditions on a shelf in a fume hood. About a month later, a Raman spectrum was recorded, which was significantly different to that recorded earlier. This is shown in FIG. 4, where the lower spectrum accords to the earlier measurement and the upper spectrum accords to the later measurement. In the light of these results, it was concluded that the prior art amorphous form of pemirolast sodium is physically unstable.

The amorphous material was also prepared by drying of the form obtained in accordance with Example 11 below at 40° C. and reduced pressure for 40 hours to yield 12 g of a pale yellow cotton-like amorphous solid.

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http://www.lookchem.com/Chempedia/Chemical-Technology/Organic-Chemical-Technology/18815.html

1) Firstly, 2-Amino-3-methylpyridine (I) is condensed with ethoxymethylenemalonodinitrile (II) to afford the monocyclic intermediate (III), which is in tautomeric equilibrium with the pyridopyrimidine derivative (IV). Next, the reaction of (IV) with aluminum azide (AlCl₃.NaN₃) in refluxing THF yields 4-imino-9-methyl-3-(1H-tetrazol-5-yl)-4H-pyrido[1,2-a]pyrimidine (V). Finally, this compound is first hydrolyzed with 1N HCl and then treated with KOH.
2) Compound (IV) can be converted to the final product by a one-pot reaction: (VI) is treated first with NaN₃ in refluxing acetic acid, then hydrolyzed with HCl and finally treated with KOH.

………….

EXAMPLE 1

A suspension of 9-methyl-3-(1 H-tetrazol-5-yl)-4H-pyrido-[1,2-a]-pyrimidin-4-one (68.5 g; 0.3 mols) in methanol (420 ml) and water (210 ml) heated at 50° C. is added with a 40% N-methylamine aqueous solution (30 ml, 0.35 mols) to pH=10. The solution is heated at 68-70° C., and acidified with formic acid (21 ml) to pH=3. After completion of the addition the mixture is kept at 68-70° C. for about 15 minutes and then cooled to 20-25° C. The precipitate is filtered, washed with methanol and dried under vacuum at 40° C. to give 9-methyl-3-(1 H-tetrazol-5-yl)-4H-pyrido-[1,2-a]-pyrimidin-4-one with >99.8% HPLC purity (63 g, 92% yield).

EXAMPLE 2

9-Methyl-3-(1 H-tetrazol-5-yl)-4H-pyrido-[1,2-a]-pyrimidin-4-one (63 g, 0.28 mols) is suspended in methanol (1000 ml). The resulting suspension is kept at 45° C. and slowly added with a 45% potassium hydroxide aqueous solution to pH 9-9.5. The suspension is stirred at 45° C. for about 15 minutes and then cooled to 20° C. The precipitate is filtered, washed with methanol and dried under vacuum at 80° C., to obtain Pemirolast Potassium (71.9 g; 0.27 mols, 96% yield) with HPLC purity >99.8%. ¹H NMR(D₂O, TMS) d (ppm): 2.02 (s, 3H); 6.83 (t, 1H); 7.22 (d, 1H); 8.18 (s, 1H); 8.47 (d, 1H).

References

• Tinkelman DG, Berkowitz RB (February 1991). “A pilot study of pemirolast in patients with seasonal allergic rhinitis”. Ann Allergy 66 (2): 162–5. PMID 1994787.
• Kawashima T, Iwamoto I, Nakagawa N, Tomioka H, Yoshida S (1994). “Inhibitory effect of pemirolast, a novel antiallergic drug, on leukotriene C4 and granule protein release from human eosinophils”. Int. Arch. Allergy Immunol. 103 (4): 405–9. doi:10.1159/000236662. PMID 8130655.
• Abelson MB, Berdy GJ, Mundorf T, Amdahl LD, Graves AL (October 2002). “Pemirolast potassium 0.1% ophthalmic solution is an effective treatment for allergic conjunctivitis: a pooled analysis of two prospective, randomized, double-masked, placebo-controlled, phase III studies”. J Ocul Pharmacol Ther 18 (5): 475–88. doi:10.1089/10807680260362759. PMID 12419098.
• Kemp JP, Bernstein IL, Bierman CW et al. (June 1992). “Pemirolast, a new oral nonbronchodilator drug for chronic asthma”. Ann Allergy 68 (6): 488–91. PMID 1610024.

Pemirolast

Systematic (IUPAC) name
9-methyl-3-(1H-tetrazol-5-yl)-4H-pyrido[1,2-a]pyrimidin-4-one
Clinical data
Trade names	Alamast
AHFS/Drugs.com	monograph
Pregnancy category	US: C (Risk not ruled out)
Legal status	℞ (Prescription only)
Routes of administration	Oral, ophthalmic
Identifiers
CAS Registry Number	69372-19-6
ATC code	None
PubChem	CID: 57697
IUPHAR/BPS	7329
DrugBank	DB00885
ChemSpider	51990
UNII	2C09NV773M
KEGG	D07476
ChEMBL	CHEMBL1201198
Chemical data
Formula	C10H8N6O
Molecular mass	228.21 g/mol

Pemirolast Potassium (BMY 26517) cas100299-08-9is a histamine H1 antagonist and mast cell stabilizer that acts as an antiallergic agent.
Target: Histamine H1 Receptor
Pemirolast potassium (BMY 26517) is a new oral, nonbronchodilator antiallergy medication that is being evaluated for the therapy of asthma [1]. Pemirolast potassium (BMY 26517) inhibits chemical mediator release from tissue mast cells and is also shown to inhibit the release of peptides including substance P, Pemirolast potassium (BMY 26517) reduces kaolin intake by inhibition of substance P release in rats [2]. Pemirolast potently attenuates paclitaxel hypersensitivity reactions through inhibition of the release of sensory neuropeptides in rats [3]. Pemirolast potassium is used for the treatment of allergic conjunctivitis and prophylaxis for pulmonary hypersensitivity reactions to drugs such as paclitaxel [4].

Molecular formula: C₁₀H₇KN₆O

Molecular Weight: 266.30

External links

• Mitsubishi Tanabe Pharma Corporation (2007). “ALEGYSAL (English)” (PDF). Retrieved 2008-09-02.
• “DailyMed Announcements”. U.S. National Library of Medicine. 2005. Retrieved 2008-09-02.