Latanoprost acid and brimonidine conjugates

A compound of formula (I),

• in which
• each of R1, R2, and R5, independently, is H, halo, or OH;
• each of R3 and R4 is H, OH, or amino, or R3 and R4, together with the phenyl ring they are bonded to, form a quinoxaline ring;
• X1 is CO, CHOH, or CF2;
• X2 is C4-C6 alkyl, C4-C6 alkyl-O—, aryl-O—, or aryl-CH2—;
• X3 is
• and
• is a single or double bond.