Nuclease-resistant RNA aptamers are provided which are capable of neutralizing PDGFRβ and are therefore useful in the diagnosis and/or therapy of PDGFRβ-associated and hyperproliferative-associated diseases, such as cancer and primary tumor metastasis. RNA aptamers provided herein include a modified synthetic RNA sequence wherein at least one pyrimidine residue is modified to 2′-fluoropyrimidine. Pharmaceutical compositions and diagnostic kits comprising RNA aptamers are also provided.