An infusion catheter system may deliver a therapeutic agent to a desired location in the body. The system may include an elongate catheter shaft having a proximal end region, a distal end region, a distal opening, and a lumen extending between the proximal end region and distal opening. The distal end region may include a coiled portion having a preformed coiled configuration including a plurality of helically wound rings. A plurality of apertures may be formed through a sidewall of the coiled portion. A guidewire may be configured to be slidably disposed within the lumen of the catheter shaft. The guidewire may include a proximal end region having a first cross-sectional dimension, an enlarged distal end region having a second cross sectional dimension greater than the first cross-sectional dimension, and an intermediate region disposed between the proximal end region and the distal end region.