In addition to what u/ChromaticDragon already mentioned, if I recall correctly, it would in theory be possible to have an atomic nucleus comprised of protons and antineutrons. However, the protons consist of 3 quarks (uud) and the neutrons consist of 3 antiquarks (-udd).

=> The atom would not be very stable, as the quarks might interact and annihilate with the antiquarks.