Limiting the current without limiting the voltage
It is not possible because of Ohm's Law:
$$V = IR$$
Your load has \$R = 500{\textrm M\Omega}\$
Therefore \$V = I \cdot 500{\textrm M\Omega}\$
If \$I\$ is 5 nA then \$V\$ is 2.5 V, because 5 nA times 500 M\$\Omega\$ is 2.5 V.
Full stop. End of story.