Why we don't have any human data on the LD50 of plutonium, it is not likely to be any more toxic than uranium which has a LD50 of around 5g for the soluble salt

Why we don't have any human data on the LD50 of plutonium, it is not likely to be any more toxic than uranium which has a LD50 of around 5g for the soluble salts. A 20g plutonium ball should be harmless if swallowed. 20g of caffeine on the other hand won't be pleasant.