Drug-conjugates, conjugation methods, and uses thereof

An active agent-conjugate comprising Formula Ia:

• or a pharmaceutically acceptable salt thereof,
• wherein:
• the A-component is an antibody or an antibody fragment;
• the E-component is selected from the group consisting of:
• L3 is an optionally substituted C1-C6 alkyl, or L3 is null, wherein when L3 is null the sulfur is directly connected to the E-component;
• L4 is an optionally substituted C1-C6 alkyl, or L4 is null, wherein when L4 is null the sulfur is directly connected to the E-component;
• B is a hydrophilic polymer, a biodegradable polymer, a polyamino acid, a polypeptide, a polysaccharide, a monoclonal antibody, an antibody fragment, a protein ligand, a protein scaffold, a peptide, or a nucleic acid, or B is null;
• each D is independently selected, wherein each D includes an active agent;
• each L2 independently comprises a —(CH2)n—, —(CH2CH2O)n—, a peptide, an oligosaccharide, or a combination thereof;
• wherein L2 links to E;
• and
• n is 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10.