Compositions and methods for treating and preventing neurodegenerative disorders

A compound having the structural formula (IV-1):

• or a pharmaceutically acceptable salt thereof, wherein
• Rb1 is selected from hydrogen, —(CH2)1-3—C(O) OH, —(CH2)1-3—C(O)O(C1-C3 alkyl), —C(O)—[CH(RA)]1-2—NH—RB, and —C(O)—[CH(RA)]1-2—NH—C(O)—[CH(RA)]1-2—NH—RB;
• each R7 and each R8 is independently selected from —H, —NH2, —NRaRb—C(O)NH2, —C(O)NRaRb, —(C(Rx)(Ry))nNH2, —(C(Rx)(Ry))n NRaRb, —(C(Ra)(Rb))nC(O)NH2, —(C(Ra)(Rb))nC(O)NRaRb, —OH, —(C(Ra)(Rb))nOH, —CO2H, —(C(Ra)(Rb))nCO2H, —SO3H, —(C(Ra)(Rb))nSO3H, deuterium, halogen, alkyl, alkoxy, alkenyl, alkynyl, cyano, cycloalkyl, heterocycloalkyl, aryl, and heteroaryl;
• each Ra is hydrogen or optionally substituted alkyl;
• each Rb is selected from hydrogen, alkyl substituted with a carboxyl or a carboxylate, and an amino acid or a dipeptide, wherein the amino acid or the dipeptide is bound to the nitrogen atom in R3 through a carboxy group;
• or Ra and Rb are taken together to form an optionally substituted heterocyclyl;
• each Rx and Ry are independently a hydrogen or a C1-C6 alkyl group
• n is 1 or 2;
• each RA is independently selected from hydrogen or a side group of a natural or unnatural amino acid;
• and
• RB is selected from hydrogen and a protecting group.