Unit structure-type pharmaceutical composition for nucleic acid delivery

1. A unit structure, consisting of:

a single double-stranded nucleic acid having 18 to 30 base pairs, and

two block copolymers, each having a cationic polyamino acid segment and a 2-arm branched hydrophilic polymer chain segment,

wherein:

(i) a difference between a total of positive charges derived from cationic groups of the cationic polyamino acid segments and a total of negative charges derived from the nucleic acid in the unit structure falls within a range of ±10% of the total of the negative charges derived from the nucleic acid,

(ii) the 2-arm branched hydrophilic polymer chain segment comprises two polyethylene glycol chains, each polyethylene glycol chain having a molecular weight of 20,000 Da or more,

(iii) each of the polyethylene glycol chains has a radius of inertia (Rg) equal to or longer than half the length of the nucleic acid, and

(iv) the cationic polyamino acid segments are bound to the nucleic acid via electrostatic bonds.

2. The unit structure according to claim 1 , wherein

all cationic amino acids in the cationic polyamino acid segment have only one cationic group in each side chain.

3. The unit structure according to claim 1 , wherein

the cationic polyamino acid segment contains exclusively cationic amino acid residues.

4. The unit structure according to claim 1 , wherein

the nucleic acid is siRNA.

5. The unit structure according to claim 4 , wherein

the polyamino acid chain segment of each block copolymer contains a number of positive charges that is at least approximately one-half of the total of the negative charges derived from the nucleic acid.

6. The unit structure according to claim 4 , wherein

each of the polyethylene glycol chains has a molecular weight of 20,000 Da to 60,000 Da.

7. A unit structure, consisting of one siRNA and two block copolymers, wherein

(i) each block copolymer has a cationic polyamino acid segment and a 2-arm branched polyethylene glycol at one terminal of the polyamino acid chain segment,

(ii) each of the arms of the polyethylene glycol has a molecular weight of from 20,000 Da to 80,000 Da,

(iii) a difference between a total of positive charges derived from cationic groups of the cationic polyamino acid segments and a total of negative charges derived from the siRNA in the unit structure falls within a range of ±10% of the total of the negative charges derived from the siRNA, and

(iv) the cationic polyamino acid segments are bound to the siRNA via electrostatic bonds.

8. The unit structure according to claim 7 , wherein

all cationic amino acids in the cationic polyamino acid segment have only one cationic group in each side chain.

9. The unit structure according to claim 7 , wherein

the cationic polyamino acid segment contains exclusively cationic amino acid residues.

10. The unit structure according to claim 7 , wherein the two block copolymers are each selected from the group consisting of the following formulae (1)-(2):

wherein:

R 1a to R 1d are each independently a hydrogen atom, an unsubstituted or substituted linear or branched alkyl group having 1 to 12 carbon atoms, or a group represented by the following formula (I):

where k represents an integer of from 1 to 5, and D represents a target binding site;

R 2 is a hydrogen atom, an unsubstituted or substituted linear or branched alkyl group having 1 to 12 carbon atoms, or an unsubstituted or substituted linear or branched alkylcarbonyl group having 1 to 24 carbon atoms;

R 3 is a hydroxyl group, an unsubstituted or substituted linear or branched alkyloxy group having 1 to 12 carbon atoms, an unsubstituted or substituted linear or branched alkenyloxy group having 2 to 12 carbon atoms, an unsubstituted or substituted linear or branched alkynyloxy group having 2 to 12 carbon atoms, or an unsubstituted or substituted linear or branched alkyl-substituted imino group having 1 to 12 carbon atoms;

R 4a and R 4b are each independently a methylene group or an ethylene group;

R 5a and R 5b are the same group or different groups selected from the group consisting of:

—NH—(CH 2 ) p1 —[NH—(CH 2 ) q1 —] r1 NH 2   (i);

—NH—(CH 2 ) p2 —N[—(CH 2 ) q2 —NH 2 ] 2   (ii);

—NH—(CH 2 ) p3 —N{[—(CH 2 ) q3 —NH 2 ][—(CH 2 ) q4 —NH—] r2 H}  (iii); and

—NH—(CH 2 ) p4 —N{—(CH 2 ) q5 —N[—(CH2) q6 -NH 2 ] 2 } 2   (iv),

where p1 to p4, q1 to 6, and r1 to 2 are each independently an integer of from 1 to 5;

Q is —NH 2 , —NHC(═NH)NH 2 , or a group represented by the following formula (II);

L is a divalent linking group or a valence bond;

x1 to x4 are each independently an integer of from 454 to 1818;

y, z, and v are each independently an integer of from 0 to 30, provided that y, z, and v satisfy the relationship 18≦y+z+v≦30;

w is an integer of from 1 to 6;

1 and m are each independently an integer of from 0 to 5; and

n is 1.

11. The unit structure according to claim 10 , wherein x1 to x4 are each independently an integer of from 454 to 1200.

12. The unit structure according to claim 11 , wherein:

the siRNA is a double-stranded RNA having 21 base pairs and

y, z, and v are selected such that the number of cationic groups in the cationic polyamino acid segment is an integer of from 18 to 22.

13. A method of treating cancer, comprising:

administering a therapeutically effective amount of the unit structure according to claim 1 to a patient in need thereof.

14. A method of treating cancer, comprising:

administering a therapeutically effective amount of the unit structure according to claim 7 to a patient in need thereof.

15. The unit structure according to claim 1 , each polyethylene glycol chain has a molecular weight of from 30,000 Da to 60,000 Da.

16. The unit structure according to claim 7 , each strand of the polyethylene glycol has a molecular weight of from 30,000 Da to 60,000 Da.