We have identified two synthetic
oligonucleotides (aptamers) that bind to
prostate cancer cells,with low nanomolar affinity, via the extracellular portion of the prostate-specificmembrane
antigen (PSMA). These two specific aptamers were selected from an initial 40mer library of approximately 6 x 10(14) random-sequence RNA molecules for their ability to bind to a
recombinant protein representing the extracellular 706
amino acids of PSMA, termed xPSM. Six rounds of in vitro selection were performed, enriching for xPSM binding as monitored by aptamer inhibition of xPSM N-acetyl-alpha-linked
acid dipeptidase (
NAALADase) enzymatic activity. By round six, 95% of the aptamer pool consisted of just two sequences. These two aptamers, termed xPSM-A9 and xPSM-A10, were cloned and found to be unique, sharing no consensus sequences. The affinity of each aptamer for PSMA was quantitated by its ability to inhibit
NAALADase activity. Aptamer xPSM-A9 inhibits PSMA noncompetitively with an average K(i) of 2.1 nM, whereas aptamer xPSM-A10 inhibits competitively with an average K(i) of 11.9 nM. Distinct modes of inhibition suggest that each aptamer identifies a unique extracellular
epitope of xPSM. One aptamer was truncated from 23.4 kDa to 18.5 kDa and specifically binds LNCaP human
prostate cancer cells expressing PSMA but not PSMA-devoid PC-3 human
prostate cancer cells. These are the first reported
RNA aptamers selected to bind a
tumor-associated membrane
antigen and the first application of
RNA aptamers to a prostate specific cell marker. These aptamers may be used clinically as
NAALADase inhibitors or be modified to carry imaging agents and therapeutic agents directed to
prostate cancer cells.