## 6 AMPs--Goods vs. Bads, and the Long Way towards Clinical Application
\"The PAEs presented here provide an additional hypothesis besides immunomodulatory effects that can explain the good in vivo efficacies of PrAMPs\", notwithstanding the fast clearance rate measured for some of these peptides, authors discussed [162], \"This again highlights that MIC values determined for AMPs in vitro cannot be simply used to predict in vivo efficacies, as often assumed in the literature. Instead, MIC values should be seen as one important criterion among other parameters to be considered,\" authors appropriately remarked [162]. One of the aspects that are often quoted in support of the (potential) use of AMPs in clinical practice is their low tendency to evoke antibiotic resistance. This tenet stems from the fact that AMPs generally (but not always, as specified above) hit the lipid component of the plasma membrane, a cellular component that is believed _per se_ to be not easily modifiable in its basic physicochemical features by microbial targets. Although the slower emergence of resistance to AMPs with respect to conventional antibiotics is a reality, however, experience and much work have clearly shown that the reassuring thought that the complex phenomenon of resistance would not eventually thwart AMPs' value, is somewhat naive and misleading. In fact, the long coevolution of microorganisms and AMPs has spurred the development of several resistance mechanisms. These include sequestration by bacterial enzymes, proteolytic degradation of peptides, efflux pumps to remove AMPs from the periplasmic space, alteration of components of bacterial surface to reduce surface attachment and permeability, down-regulation by immunomodulation [163; 164; 165; 166]. The concept of coevolution and its effect on the rise of bacterial resistance to AMPs' action is well explained by the example of _Helicobacter pylori_. [PERSON] and colleagues tested the pattern of induction of gastric antimicrobial peptides by _H. pylori_ as well as its susceptibility to the same peptides [167]. Researchers found that the induction of antimicrobial peptides, such as the inducible defensin HBD2 in the gastric mucosa by _H. pylori_, did not enhance the killing capacity against _H. pylori_ itself. On the other hand, the expression levels of the constitutive defensin HBD1, inducible HBD3 and LL37, remained unchanged. Tested _H. Pylori_ strains proved resistant to HBD1, but susceptible to the killing activities of HBD3 and LL37. \"The combination of selective defensin induction and resistance to others may enable _Helicobacter_ to colonize the gastric mucus layer where it can adhere to epithelial cells and induce inflammatory as well as malignant processes,\"concluded the authors, that remarked the need for further studies aimed at understanding the mechanisms regarding _H. pylori_ selective antimicrobial resistance [168]. Despite the limitations briefly outlined above, that have hampered their development in the classical drug discovery pipeline, AMPs are attracting continuous and ever-increasing interest as new antimicrobials agents. Out of some \(\sim\)3000 molecules that have been isolated from different sources, just a handful have been the object of preclinical studies and further proceeded to clinical trials [166]. A recent analysis of AMPs patents from 2015 through 2020 has confirmed a long-standing trend, i.e., the fact that AMPs earmarked for clinical development are in vast majority analogs or derivatives of natural peptides, obtained through a template-based strategy aimed at enhancing the activity and stability of natural AMPs while reducing their toxicity [168]. Currently, just three AMPs have been approved by the U.S. Food and Drug Administration (FDA) for therapeutic use, i.e., gramicidin, colistin and daptomycin. Gramicidin has a long history. First isolated from _Bacillus brevis_ over 70 years ago, gramicidin is active against a range of Gram-positive and Gram-negative bacteria, although its severe toxicity for human erythrocytes has a limited clinical indication to topical applications [169].