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Since the introduction of recombinant insulin in 1979 and thanks to the advances in pharmaceutical biotechnology, therapeutic proteins and peptides have gained significant importance in the treatment of many diseases and after they represent the only therapeutic option. However, such growing success is not devoid of potential pitfalls like short half-life, easy enzymatic degradation and immunogenicity. PEGylation, namely the attachment of polyethylene glycol (PEG) chain(s) to a bioactive molecule, is nowadays one of the most commonly used strategies to overcome the inherent limits of therapeutic proteins. The studies reported in this thesis are focused on the design and preparation of conjugates of therapeutic proteins and peptides with optimized polymeric carriers, in order to obtain improved performances in both pharmacokinetic and pharmacodynamic profiles of a conjugated biologic. The proteins and the peptide used are recombinant human granulocyte colony-stimulating factor (G-CSF), recombinant human growth hormone (hGH) and KR14, an anti-HIV triazole peptide (PT). G-CSF, hGH and KR14, owing to their low molecular weights, are rapidly excreted from the body and are subject to proteolytic degradation, resulting in low bioavailability and therefore requiring frequent administration schedule. The first part of this PhD thesis was focused on: i) transglutaminase-mediated conjugation of common non charged or new multicarboxylic PEGs to G-CSF, ii) characterization and stability studies of the conjugates, iii) pharmacokinetic studies in animals, to allow the comparison between the conjugates and with the reference G-CSF. In this study, the aim was to test multicarboxylic PEG derivatives for evaluating the contribution of negative charges on protein half-life prolongation thanks to the charge-selective property of the glomerular filtration barrier. PEGylation with high molecular weight PEGs, by increasing the hydrodynamic volume of a protein, decreases the glomerular filtration rate, but may constitute a steric hindrance and reduce the binding affinity between the therapeutic protein and its target. The charge selectivity concept of the glomerular filtration barrier can be exploited with the new polyanionic polymers, which with low molecular weights, can still extend the PK profile of the protein and achieve also a better preservation of protein activity. The investigated polyanionic PEG derivatives were i) H2N-PEG5k-(COOH)7, a heterobifunctional PEG that was derivatized on one end with β-glutamic acid units to contain 7 carboxyl groups and ii) H2N-PEG3.4k-b-PLE50, a linear polyanionic polymer that consists of a PEG linked to a PLE with the pendant ϒ-carboxyl groups of glutamic acid monomers. In addition to these two multicarboxylic PEGs, two classic uncharged PEGs with molecular weights of 5 kDa and 20 kDa were also used. After PEGylation, all the G-CSF-PEG conjugates were characterized by MALDI-TOF mass spectrometry, SDS-PAGE and circular dichroism and the pharmacokinetic studies were carried out in rats. The second part of this PhD thesis focused on the comparison of the biological activity (stimulation of somatic growth) of three hGH conjugates differing for the polymeric carrier (multicarboxylic or neutral) and for the site of polymer conjugation. The aim of this study was to evaluate if the protein stability and the protein/receptor recognition were influenced by the site of conjugation of the polymer and to assess the impact of polyanionic carriers on the pharmacodynamic behaviour of the protein. Conjugation of the multicarboxylic polymer, poly(L-glutamic acid), resulted in an increase of the final negative charge of the conjugate. Based on the charge selectivity concept of glomerular filtration, polyanionic polymers having relatively low molecular weights may provide a half-life extension of a given protein, similar to that provided by higher molecular weight neutral polymers. Two different site-specific mono-PEGylated forms of hGH were prepared exploiting an enzymatic PEGylation (PEG-Gln141-hGH) via transglutaminase (TGase) and a chemical N-terminal PEGylation (PEG-Nter-hGH), using neutral 20kDa PEGs. Interestingly the conjugates showed an increased thermal stability and the ability to refold after thermal denaturation. The third monoconjugate of hGH was prepared by chemical N-terminal conjugation, using poly(L-glutamic acid) with 50 glutamic acid units and with an aldehyde function at one end (PLE50ald). The glutamic acid monomers confer to this polymer a polyanionic characteristic. hGH-PLE50ald was characterized by MALDI-TOF mass spectrometry, SDS-PAGE, circular dichroism and the pharmacokinetic studies were carried out in rats. The bioactivities of a single dose of hGH-PLE50ald, PEG-Gln141-hGH and PEG-Nter-hGH were similar or even better to that of a daily injection of hGH, by testing the somatic growth in hypophysectomized rats. The third section of this work has dealt with: synthesis and characterization of KR14 peptide and its conjugation with PEG20kDa-Mal, pharmacokinetic studies of the conjugate in mice and SPR affinity binding experiments. A part of this work was developed at the Department of Biochemistry and Molecular Biology of the Drexel University College of Medicine in Philadelphia, under the supervision of Professor Irwin Chaiken, whose laboratory is focused on the design and realization of peptide drugs that inhibit the entry of HIV-1 in the target cells. This initial step of HIV-1 entry is characterized by the binding of envelope glycoprotein gp120 on to the cellular receptor CD4 and co-receptor. The triazole peptide KR14 derive from a previously known family of PT’s that has great therapeutic potential as it exhibits high affinity binding to gp120 and inhibits interactions of gp120 with both CD4 and the co-receptor surrogate mAb17b. KR14 was designed to contain a free thiol group at position 16 that has been exploited to achieve selective PEGylation with a PEG-maleimide of 20 kDa. The identity and purity of the conjugate were confirmed by SEC-HPLC and MALDI-TOF. Pharmacokinetic studies in mice demonstrated that PEGylation extended the t1/2 of the peptide from 44 min to 289 min. Even more remarkably the conjugate did not show the typical loss in bioactivity, common to most PEGylated drugs. In fact SPR studies demonstrated that PEG-KR14 is only about 1.5, 1.6 fold less active than KR14, a relevant result. In conclusion the studies of this PhD thesis further demonstrated the potential of PEGylation in the delivery of protein and peptides. Although PEGylation is already a mature technology that yielded several products in clinical use, we demonstrated that there are still reasons for implementation and new discoveries by pursing also the renal filtration charge selectivity and not only the size selectivity as done so far in this field.