The Melodic Minor Pentatonic is considered a modal pentatonic scale.
A modal pentatonic scale is created when two notes are removed from a seven note scale.
The five notes left over are usually chord tones plus one or two characteristic sounding notes from the original scale.
In this case our original scale is Melodic Minor so we chose the Root, second, minor third, perfect fifth, and major sixth as our notes.
Another version of Melodic Minor Pentatonic adds the minor seventh in place of the second.