A translation of the Latin inscription found at the bottom of the painting roughly reads as   follows: 

Visible and vigorous man Matthew Bologninus, Pavia castle commander created holy angels companion Francesco Sforza Duke of Milan in 1450, demonstrate ties, St. John the Evangelist, and Anthony Abbot, from the Mother the client received.

The two saints are easily identified by the faint lettering of their names: Saint John the Evangelist on the left and Saint Anthony Abbot on the right.  The man in the bottom left hand corner is the ‘donor,’ Francesco Sforza who succeeded his father in law Filippo Maria Visconti as Duke of Milan.  It is likely that the piece was commissioned either by Francesco during his time as duke, from 1450-1466, but could have been commissioned by his son Galaezzo Maria Sforza who followed as Duke of Milan from 1466-1476 and was the patron of many Visconti-Sforza portraits and took a special interest in the decoration of Pavia.  Although it is unclear weather or not the Madonna and Child with St. John the Evangelist, a Donor, and St. Anthony Abbot was painted by one of the Sforza court artists, it seems a reasonable possibility that it was.  If this is was indeed the case, it can possibly be attributed to Bonifazio Bembo, a court painter who did portraits of the Visconti-Sforza lineages including one cycle featuring the Madonna and Child with the family.