3,6-disubstituted xanthylium salts and use thereof in treatment of tauopathies

A method of treatment of a tauopathy condition or a disease of tau protein aggregation in a patient in need thereof comprising administering to the patient a compound of formula (I):

• wherein:
• X− is a counter ion;
• —R5 is independently —H, or saturated C1-6alkyl, which is unsubstituted or substituted with one or more substituents —R5A, or phenyl, which is unsubstituted or substituted with one or more substituents —R5A;
• each —R5A is independently selected from —F, —Cl, —Br, —I, —OH, —OR6, —SH, —SR6, —CN, —NO2, —NH2, —NHR6, —NR62, —NHC(═O)R6, —NR6C(═O)R6, —C(═O)OR6, —OC(═O)R6, —C(═O)NH2, —C(═O)NHR6, and —C(═O)NR62, —C(═O)R6, —C(═O)OH, —S(═O)R6, —S(═O)2R6, and —S(═O)2OH;
• each —R6 is independently saturated aliphatic C1-4alkyl, phenyl, or benzyl;
• —R13a, —R13b, —R14a, —R14b, —R15a, —R15b, —R16a, and —R16b are each independently selected from H and saturated aliphatic C1-4 alkyl.