CMOS pixel sensors are foreseen to equip the vertex detector (called PXL) of the upgraded inner tracking system of the STAR experiment at RHIC. The sensors (called ULTIMATE) are being designed and their architecture is being optimized for the PXL specifications, extrapolating from the MIMOSA-26 sensor realized for the EUDET beam telescope. The paper gives an overview of the ULTIMATE sensor specifications and of the adaptation of its forerunner, MIMOSA-26, to the PXL specifications. One of the main changes between MIMOSA-26 and ULTIMATE is the use of a high resistivity epitaxial layer. Recent performance assessments obtained with MIMOSA-26 sensors manufactured on such an epitaxial layer are presented, as well as results of beam tests obtained with a prototype probing improved versions of the MIMOSA-26 pixel design. They show drastic improvements of the pixel signal-to-noise ratio and of the sensor radiation tolerance with respect to the performances achieved with a standard, i.e. low resistivity, layer.