Measurement of Cetuximab and Panitumumab-Unbound Serum EGFR Extracellular Domain Using an Assay Based on Slow Off-Rate Modified Aptamer (SOMAmer) Reagents

Abstract

BackgroundResponse to cetuximab (Erbitux®) and panitumumab (Vectibix®) varies among individuals, and even those who show response ultimately gain drug resistance. One possible etiologic factor is differential interaction between the drug and target. We describe the development of an assay based on Slow Off-rate Modified Aptamer (SOMAmer™) reagents that can distinguish drug-bound from unbound epidermal growth factor receptor (EGFR).MethodsThis quantitative assay uses a SOMAmer reagent specific for EGFR extracellular domain (ECD) as a capturing reagent. Captured SOMAmer is quantitated using PCR. Linearity and accuracy (recovery) of the assay were assessed using normal sera and purified EGFR ECD.ResultsThis EGFR ECD assay showed linearity between 2.5 and 600 ng/mL. Average recovery was 101%. The assay detected EGFR but showed little cross-reactivity to other ErbB proteins: 0.4% for ErbB2, 6.9% for ErbB3, and 1.3% for ErbB4. Preincubation of normal serum with either cetuximab or panitumumab resulted in a dose-dependent decrease in EGFR ECD levels measured using the SOMAmer assay; preincubation did not affect measurement with an ELISA.ConclusionsThis SOMAmer-based serum EGFR ECD assay accurately and specifically measures EGFR in serum. Detection of significant amounts of drug-unbound EGFR in patients undergoing cetuximab or panitumumab treatment could be an indicator of poor drug response. Further studies are needed to evaluate the utility of the assay as an indicator of drug efficacy or as a guide to dosing.

Chemical Modifications of Nucleic Acid Aptamers for Therapeutic Purposes

“…In this review, the standard synthetic method of solid phase phosphoramidite chemistry for nucleic acid aptamers preparation will be introduced firstly [18,19]. Then, the chemical modification strategies of aptamers for resisting nuclease degradation [12,[20][21][22][23][24][25][26], improving target binding affinities [10,[27][28][29][30][31] and resisting renal clearance [32][33][34][35][36] will be summarized, sequentially. Among the modifications, such as modifications on the terminals of nucleic acids, modifications on the phosphodiester linkage, modifications on the sugar ring and modifications on the bases, the 3′ end capping with inverted thymidine [6,12] and PEGylation [13] have been the common strategies in the chemical modifications of nucleic acid aptamers for development clinical therapeutics (e.g., pegaptanib [14][15][16][17], etc.).…”

Aptamers as Diagnostic Tools in Cancer

“…One such recent application using modified oligonucleotides has made aptamers even more specific and useful [ 142 ]. SomaLogic (Boulder, CO, USA) has been generating SOMAmers (slow off-rate modified aptamers) through the incorporation of modified ribose sugars to the aptamer backbone in order to enhance functionality and streamline post-selection optimization [ 143 , 144 ]. The future of aptamer-based cancer diagnosis and biomarker discovery seems to be close to SomaLogic’s development [ 145 ].…”

Aptamer Nanomedicine for Cancer Therapeutics: Barriers and Potential for Translation

“…The modified single-stranded DNA library has been used to generate aptamers against protein targets where unmodified single-stranded DNA candidates have failed, with an overall success rate as high as 84% in a large-scale selection. 43 This type of aptamer is also known as SOMAmer (slow off-rate modified aptamer) and is now applied to cancer diagnostics 47 and biomarker discovery for diseases such as chronic kidney disease 43 and lung cancer. 48 Another similar approach that enhances base hydrophobicity and improves the diversity of the aptamer library was reported by Kimoto et al 46 A hydrophobic but artificial base (7-(2-thienyl)imidazo [4,5-b]pyridine) was added to the candidate sequences.…”