Check out the two roads leaving Andoas; one from just north-west of the airport and the other heading east of the city. Unfortunately the resolution isn’t the greatest on Google Maps, so you might want to look at it in Google Earth.
While the eastern road goes to the Rio Tigre, the north-west/west route runs down to the border of Loreto and Amazonas where it then crosses the Marañon and proceeds south for a bit. From there, it runs east staying just a little north of the Marañon (becoming the Ruta Nacional 5NC) until it hits San José de Saramuró. It then runs up the Corrientes to Trompeteros.