Running an AP Pantheon is going to be difficult at best, and probably not viable. His most commonly used abilities both rely on AD, which also boosts the efficacy of his autoattacks as well. Without that AD it's much harder to be an effective harass in the lane, as your Spear Shot isn't as threatening. On top of that the ROI for AD is higher than AP for his various abilities, giving you diminished bang for your buck.
If that hasn't dissuaded you, consider instead going a hybrid route, building yourself Sheen/Phage/Zeal and upgrading into a Trinity Force since it benefits all your abilities. Additional AD/AP items to consider would be Guinsoo's Rageblade, Hextech Gunblade, and Lichbane.
If you're still married to the idea of pure AP it's probably best to go with the usual suspects while throwing in some CDR to get the most out of your shield and your skyfall. Bear in mind that if you're focusing on Shield and Skyfall that means you're going to be right next to the enemy champs a lot, and this is not an ideal position to be in, so you'll need to consider defensive items to stay alive.
Normal AP items:
Rylai's Crystal Scepter
More specific to what you're asking:
Abyssal Scepter gives you AP, MPen, and some MR to increase your survivability
Malady gives you AP and increased attack speed, and reduces their MR
Moonflair SpellBlade can be used as your Tenacity item so you can use Ionian Boots for the CDR
Morello's Evil Tome has great CDR
Nashor's Tooth has a little bit of everything you want, AP, Speed, Mana regen, and CDR
Rylai's Crystal Scepter cna be paired with Spear Shot and Heartseeker to slow them down a bit
Zhonya's Hourglass gives you a boatload of AP and some survivability
Just remember you're also going to need mana regen runes/masteries. Good luck, because I feel like you're going to need it running this build :-p