# Moving towards Personalized Medicine--The Broad Use of Aptamers for Targeted Theranostic  
[PERSON]  
1 HE-FP--Hospital Fernando Pessoa, CECLIN--Center of Clinical Studies, 4420-096 Gondomar, Portugal; [EMAIL_ADDRESS] (A.P.S. ); [EMAIL_ADDRESS] (A.C.R. ); [EMAIL_ADDRESS] (C.A. ); [EMAIL_ADDRESS] (P.P.P. ); [EMAIL_ADDRESS] (R.F. ); [PERSON]SS] (P.B. ); [EMAIL_ADDRESS] (A.G.); [EMAIL_ADDRESS] (S.R. ); [EMAIL_ADDRESS] (D.M.-M.); [EMAIL_ADDRESS] (P.B.) [PERSON]  
1 HE-FP--Hospital Fernando Pessoa, CECLIN--Center of Clinical Studies, 4420-096 Gondomar, Portugal; [EMAIL_ADDRESS] (A.P.S. ); [EMAIL_ADDRESS] (A.C.R. ); [EMAIL_ADDRESS] (C.A. ); [EMAIL_ADDRESS] (P.P.P. ); [EMAIL_ADDRESS] (R.F. ); [PERSON]SS] (P.B. ); [EMAIL_ADDRESS] (A.G.); [EMAIL_ADDRESS] (S.R. ); [EMAIL_ADDRESS] (D.M.-M.); [EMAIL_ADDRESS] (P.B.) [PERSON]  
1 HE-FP--Hospital Fernando Pessoa, CECLIN--Center of Clinical Studies, 4420-096 Gondomar, Portugal; [EMAIL_ADDRESS] (A.P.S. ); [EMAIL_ADDRESS] (A.C.R. ); [EMAIL_ADDRESS] (A.C.); [EMAIL_ADDRESS] (P.P. ); [EMAIL_ADDRESS] (R.F. ); [EMAIL_ADDRESS] (P.B. ); [EMAIL_ADDRESS] (A.G.); [EMAIL_ADDRESS] (S.R. ); [EMAIL_ADDRESS] (D.M.-M.); [EMAIL_ADDRESS] (P.B.) [PERSON]  
3 IF-JID--Instituto de Investigacao e Innovacao em Saude, 4200-135 Porto, Portugal; [PERSON]S.R. ); [EMAIL_ADDRESS] (S.R. ); s.  
RNA sequences that would specifically recognize target molecules. Since then, many improvements have been made in this procedure to increase its selectivity and shorten the selection process [5]. Aptamers possess a vast field of applications, including diagnosis, therapy and drug delivery systems, food safety, and environment monitoring [1; 2; 6]. The conventional SELEX involves subsequent cycles of incubation, binding, partitioning, and amplification (Figure 1). It usually starts with a synthetic pool of ssDNA or RNA oligonucleotides called the primary library, which can comprise up to \(10^{15}\) different sequences. This initial pool is incubated with the target molecule (in a free or immobilized form), under specific conditions of incubation. Unbound or weakly bounded sequences are eluted (partitioning) from the target and the target-bounded sequences are then retrieved and amplified by PCR or RT-PCR, for DNA or RNA libraries, respectively. The process is repeated for several rounds (between four and 20), with increasingly stringent conditions, in order to obtain a final pool with low heterogeneity and high affinity of sequences is achieved. The enriched oligonucleotides obtained are finally sequenced and their binding affinities to the target are assessed to find the best sequence [1; 2]. The affinity of aptamer-target complex can be characterized by label-free or label-based strategies, through the calculation of dissociation constant (Kd), conformational changes, kinetics, and thermodynamics [7]. Aptamers can exhibit high affinity and low Kd (usually in pico- to nanomolar range) [2], with a similar or even superior target binding affinity than antibodies. In fact, they are often mentioned as chemical antibodies and offer several inherent advantages, as they can be produced over a wide range of targets, display low toxicity and immunogenicity, low molecular weight, good solubility and tissue permeability, high tolerance to environmental conditions (temperature, pH), conformational flexibility, easy modification and conjugation with other molecules, as well as an economical and reproducible synthesis, avoiding animals' dependency for production. Once an aptamer is selected and characterized, it can be produced at a large scale, at low cost, and with low batch inter variability [1; 2]. Therefore, numerous aptamers have been selected and applied for cancer, immune, and microbiota research. In this review, the use of aptamers in research was assessed, especially in the area of cancer, immune activation, and microbiota. Figure 1: SELEX usually starts with the incubation of the target molecule and a primary library composed of ssRNA/ssDNA oligonucleotides under specific conditions.