Original: He commissioned work on the old building from Leone Battista Alberti and Bernardo Rossellino and also got Rossellino to design a plan for an entirely new basilica, or an extreme modification of the old.
Simplified: Pope Nicholas V, (1447 - 55), was worried about it and got two architects, Leone Battista Alberti and Bernardo Rossellino, to make plans to restore it or build a new one.