Variant target binding agents and uses thereof

1. A variant target binding agent conjugate, comprising:

a binding region comprising an antibody Fv region or an antigen binding fragment thereof that specifically binds to a target antigen;

at least a portion of a Fc region of a human immunoglobulin IgG1 constant region comprising an introduced cysteine residue at amino acid position 239, that is S239C, according to the EU index as set forth in Kabat;

a therapeutic agent conjugated directly or via a linker to the introduced cysteine residue wherein the therapeutic agent exerts a cytotoxic or cytostatic effect on a cell expressing the target antigen.

2. The variant target binding agent of claim 1 , wherein the binding region specifically binds to CD70.

3. The variant target binding agent conjugate of claim 1 , wherein the therapeutic agent is conjugated to the introduced cysteine residue via a linker.

4. The variant target binding agent conjugate of claim 1 , wherein the therapeutic agent is a cytotoxic agent that exerts a cytotoxic effect on a cell expressing the target antigen.

5. The variant target binding agent conjugate of claim 4 , wherein the cytotoxic agent is conjugated to the introduced cysteine residue via a linker comprising a maleimide group.

6. The variant target binding agent conjugate of claim 1 , wherein the therapeutic agent is a DNA minor groove binding agent.

7. The variant target binding agent of claim 6 , wherein the DNA minor groove binding agent is a DNA minor groove alkylating agent.

8. The variant target binding agent conjugate of claim 1 , wherein the therapeutic agent is an anti-tubulin agent.

9. The variant target binding agent conjugate of claim 8 , wherein the anti-tubulin agent is an auristatin.

10. The variant target binding agent conjugate of claim 9 , wherein the auristatin is MMAF or MMAE.

11. The variant target binding agent conjugate of claim 1 , wherein the therapeutic agent is an enediyne, a lexitropsin, a duocarmycin, a taxane, a puromycin, a dolastatin, a maytansinoid, or a vinca alkaloid.

12. The variant target binding agent conjugate of claim 1 wherein the variant target binding agent conjugate is an intact antibody conjugated to a therapeutic agent.

13. A composition comprising a variant target binding agent conjugate comprising:

a binding region comprising an antibody Fv region or an antigen binding fragment thereof that specifically binds to a target antigen;

at least a portion of a Fc region of a human immunoglobulin IgG1 constant region comprising an introduced cysteine residue at amino acid position 239, that is S239C, according to the EU index as set forth in Kabat;

a therapeutic agent conjugated directly or via a linker to the introduced cysteine residue wherein the therapeutic agent exerts a cytotoxic or cytostatic effect on a cell expressing the target antigen; and at least one pharmaceutically compatible ingredient.

14. The composition of claim 13 , wherein the therapeutic agent is a cytotoxic agent that exerts a cytotoxic effect on a cell expressing the target antigen.

15. The composition of claim 13 , wherein the cytotoxic agent is conjugated to the introduced cysteine residue via a linker comprising a maleimide group.

16. The composition of claim 13 , wherein the therapeutic agent is a DNA minor groove binding agent.

17. The composition of claim 16 , wherein the DNA minor groove binding agent is a DNA minor groove alkylating agent.

18. The composition of claim 13 , wherein the therapeutic agent is an anti-tubulin agent.

19. The composition of claim 17 , wherein the anti-tubulin agent is an auristatin.

20. The composition of claim 13 wherein the variant target binding agent conjugate is an intact antibody conjugated to a therapeutic agent; and at least one pharmaceutically compatible ingredient.

21. The composition of claim 20 comprising an average of two therapeutic agents conjugated to each antibody.