*2.4. Other Peptides*

Polycationic peptides have been studied in recent years for their antimicrobial activity. Buforin II and ranalexin are polycationic peptides derived from amphibian tissues. Cerioni et al. investigated the efficacy of ranalexin and buforin II in the prevention of vascular prosthetic graft infection due to *Staphylococcus epidermidis* with intermediate glycopeptide resistance.

Both peptides demonstrate strong in vitro activity. In vivo studies demonstrated that buforin II and ranalexin (from 4.9 × 106 to 1.9 × 102 CFU/mL) had a stronger activity than vancomycin (from 4.9 × <sup>10</sup><sup>6</sup> to 6.2 × <sup>10</sup><sup>3</sup> CFU/mL) and teicoplanin (from 4.9 × 106 to 5.1 × 104 CFU/mL). In particular, buforin II was able to inhibit bacterial growth completely [29].

Another study compared the activity of the same polycationic peptides to that of rifampicin in the prevention of methicillin-susceptible and methicillin-resistant *Staphylococcus epidermidis* vascular prosthetic graft infections. This study found that polycationic activities against *Staphylococcus epidermidis* were comparable to rifampicin. The combinations of buforin II and ranalexin-coated grafts with cefazolin showed stronger activity against the methicillin-resistant strain (no evidence of infection) than that of the combination of rifampicin-coated grafts and cefazolin (10<sup>2</sup> bacterial growth) [30].

Pal-Lys-NH2 and Pal-Lys-Lys are lipopeptides with bactericidal activity, and they are effective against different Gram-positive cocci [31].

A study investigated their action alone or in combination with vancomycin in the prevention of prosthesis biofilm in a subcutaneous rat pouch model of staphylococcal vascular graft infection. The results of this study showed that vancomycin (from 6.94 log to 3.65 log CFU/mL) and lipopeptides (from 6.94 log to 3.87 log CFU/mL for Pal-Lys-Lys NH2 and from 6.94 log to 4.080 log CFU/mL for Pal-Lys-Lys) when used alone had similar activity. The combination of vancomycin with Pal-Lys-Lys-NH2 had the strongest efficacy (from 6.94 log to 1 log CFU/mL). The in vitro study globally confirms the in vivo one [32].

Distinctin is an antimicrobial peptide with a heterodimeric structure. It has a lytic activity on unilamellar vesicles, suggesting their possible action on bacterial membranes [33].

In a study, the efficacy of distinctin was assessed in the treatment of *Staphylococcus aureus* CVC-associated infection, in particular in inhibiting the attachment of *S. aureus* to CVCs and in increasing its susceptibility to glycopeptides and carbapenems once it is adherent. The in vitro study showed a valid activity of distinctin on the biofilm and the ability to enhance the efficacy of antibiotics when used in combination. In vivo studies confirmed these results; furthermore, treatment with antibiotics and distinctin induced a significant reduction in bacterial loads on the CVC (from 106 to 10<sup>1</sup> CFU/mL) with no evidence of bacteriemia [34].

Protegrins are cysteine-rich AMPs and comprise 16–18 amino acids. IB-367 is a synthetic protegrin with bactericidal and fungicidal activity [35].

Ghiselli et al. evaluated the efficacy of a pre-treatment with IB-367 and its capacity for enhancing the efficacy of linezolid on Gram-positive biofilm in animal models of CVC infection. The study showed that IB-367 pre-treatment of CVC enhanced linezolid activity, causing a higher biofilm bacterial load reduction (from 106 to 10<sup>1</sup> CFU/mL) and absence of bacteriemia. In conclusion, IB-367 could be considered an interesting adjunctive agent to conventional antibiotics for the treatment of CVC and other medical devices [36].

Cirioni et al. investigated the efficacy of daptomycin and rifampicin alone and in combination in the prevention of vascular graft biofilm formation in a rat pouch model of Staphylococcal infection. Rifampicin is a glycopeptide antibiotic, while daptomycin is a lipopeptide. The results of this study showed that both rifampicin and daptomycin have good efficacy when used alone (from 7.4 × <sup>10</sup><sup>6</sup> to 3.3 × <sup>10</sup><sup>2</sup> CFU/mL for daptomycin and from 7.4 × <sup>10</sup><sup>6</sup> to 4.2 × <sup>10</sup><sup>3</sup> CFU/mL for rifampicin). When they are used in combination, their efficacy is higher than that of each single compound (from 7.4 × 106 to 10<sup>1</sup> CFU/mL). Moreover, their combination prevented the emergence of rifampicin resistance in adherent bacteria. These results were confirmed by in vitro studies [37].

Another study investigated the efficacy of levofloxacin, cefazolin, and teicoplanin in preventing vascular prosthetic graft infection induced by methicillin-susceptible and methicillin-resistant *Staphylococcus epidermidis*. The three compounds had similar efficacies, but levofloxacin (from 106 to 103 CFU/mL) showed slightly less efficacy than teicoplanin (from 10<sup>6</sup> to 10<sup>2</sup> CFU/mL) against the methicillin-resistant strain. Furthermore, the results highlighted that the most useful combination for the prevention of late-appearing vascular graft infections is rifampicin-levofloxacin (no infection detected). However, rifampicinteicoplanin was also very effective (no infection was detected) [38].AMPs, in our experience, have a high efficacy in reducing bacterial load on the surface of medical devices; this efficacy is frequently comparable to that of the most commonly used antibiotics. Furthermore, when AMPs are used in combination with other antibiotics, they increase their efficacy, leading to no evidence of bacteriemia in most cases.

### **3. AMPs and Gram-Positive Sepsis**

Sepsis represents a serious clinical problem given its severity, prevalence, and difficulty in treatment. Specifically, in 50% of cases, sepsis results from Gram-positive infections. The most frequently involved microorganisms are *Staphylococcus aureus* and *Streptococcus epidermidis*. Antibiotic therapy is not always effective, partly because of the increasing prevalence of antibiotic resistance. For this reason, new molecules such as antimicrobial peptides are increasingly being considered [39].
