This started as a comment but got too big...
I suggest 2 solutions:
- A solid state relay
- Completely control the voltage across the load so you don't care about leakage of the switch devices. Either with a logic device or 2 discrete FETs
As @peufeu says you will have a hard time finding a discrete MOSFET whose datasheet guarantees such low off state leakage. Smaller area, and hence higher Ron devices will tend to have lower leakage.
Using a solid state relay (a.k.a. opto-relay), which internally uses a pair MOSFETS as the switch devices, you can find components that will guarantee the leakage is less that 25nA. However, opto-relays will be slow so don't use this for switching fatser than about 1 kHz (see the opto-relay's data sheet). If have a connection to VDD use the approach below.
Another solution is to drive the output of the with 2 MOSFETs just like the output of a CMOS inveter. Assuming the load is between a positive supply, VDD, and a negative supply, VSS, and you are controlling the load's negative terminal.
- Turn on an NMOS to turn on the load by pulling the load's negative terminal to VSS. The voltage across the load is VDD - VSS.
- Turn on a PMOS to turn off the load by pulling the load's negative terminal to VDD. The voltage across the load is VDD - VDD = 0.
If VDD - VSS is about 18 V or less, don't use discrete FETs, use a logic device, the CD family devices will handle 18V and for lower voltages use a more recent family. Otherwise, use discrete devices.
If VDD - VSS is greater than the gate breakdown of your FETS (20 V is typical) or greater than the logic swing of the driver, you will have to use separate the gate signals and level shift the gate drive to the PMOS device. Low to medium speed (<1 MHz) level shifters using resistors or resistors and a matched pnp BJT pair only require a few components and won't burn too much power (unless VDD-VSS is high).