b-lattamasi
Penicillinasi-cefalosporinasi-carbapenemasi
b-lactamase
Clinically isolated distinct b-lactamase now
number > 1400.
The simplest classification is by protein sequence,
four classes, A, B, C, and D,
Classes A, C, and D include enzymes that
hydrolyze their substrates by forming an acyl
enzyme through an active site serine, whereas
class B b-lactamases are metalloenzymes that
utilize at least one active-site zinc ion to facilitate
b-lactam hydrolysis.
Two major classification schemes exist for categorizing -lactamase enzymes: Ambler classes A through D, based on
amino acid sequence homology, and Bush-Jacoby-Medeiros groups 1 through 4, based on substrate and inhibitor profile.
A “family portrait” reveals the structural similarity of class A, C, and D serine -lactamases . Class B -lactamases (“a class
apart”) are metallo—lactamases (MBLs) .
Carbapenemases are β-lactamases that hydrolyze penicillins, in most
cases cephalosporins, and to various degrees carbapenems and
monobactams (the latter are not hydrolyzed by metallo-β-lactamases).
b-lactamase
ClassA enzymes
ClassA enzymes are very prevalent among bacteria, especially in the
Enterobacteriaceae. TEM-1 and TEM-2 were some of the first betalactamases described in the 1960s. SHV-1 was discovered later. These
enzymes can be encoded on conjugative plasmids, which have served as
genetic vehicles for wide dispersal of these resistance genes. SHV-1 is also
found on the chromosome of Klebsiella pneumoniae. TEM-1, TEM-2, and
SHV-1 confer resistance to commonly used antibiotics, such as ampicillin
and amoxicillin. Other important substrates for these three enzymes
include carbenicillin, pipercillin, cefazolin, and cefuroxime. The extent of
hydrolysis for these drugs can vary depending upon the quantity of the blactamase that is produced. Class A enzymes are inhibited by clavulanic
acid (sulbactam, tazobactam). In addition, clavulanic acid has proved
useful in the laboratory for detection of some classA enzymes.
b-lactamase
ClassA ESBL enzymes
ESBLs hydrolyze penicillins, narrow- and extended-spectrum
cephalosporins (including the anti-methicillin-resistant S. aureus
[MRSA] cephalosporin), and the monobactam aztreonam. In
contrast, ESBLs cannot efficiently degrade cephamycins,
carbapenems, and b–lactamase inhibitors. More than 200
different ESBLs have been identified, posing a significant risk to
public health and to hospitalized patients in intensive care units,
where infection with an ESBL may lead to significant morbidity
and mortality The majority of ESBLs are from the SHV, TEM,
and CTX-M families; less frequently they are derived from BES,
GES-1, VEB, and PER enzymes, and sometimes these enzymes
do not belong to any defined family.
b-lactamase
Class A serine carbapenemases
Class A serine carbapenemases include the nonmetallocarbapenamase of
class A (NMC-A), IMI, SME, and KPC. Members of this group of blactamases can hydrolyze carbapenems as well as cephalosporins,
penicillins, and aztreonam. These carbapenemhydrolyzing enzymes have
been identified primarily in Enterobacter cloacae, Serratia marcescens, and K.
pneumoniae, bacteria which often harbor multiple resistance determinants,
narrowing the range of treatment options. The bla gene for the former two
organisms is typically found on the chromosome, while the K. pneumoniae
carbapenemase blaKPC gene is carried on plasmids containing Tn4401.
MICs of carbapenems in carbapenemase-expressing strains can vary from
moderately increased (2 to 4 g/ml) to resistant ( 32g/ml)
b-lactamase
Class B metallo-b-lactamases
Class B enzymes are Zn- dependent b-lactamases that demonstrate a
hydrolytic mechanism different from that of the serine b-lactamases of
classes A, C, and D. Organisms producing these enzymes usually exhibit
resistance to penicillins, cephalosporins, carbapenems, and the clinically
available b-lactamase inhibitors . Interestingly, the hydrolytic profile of
MBLs does not typically include aztreonam. MBLs likely evolved
separately from the other Ambler classes, which have serine at their active
site. The blaMBL genes are located on the chromosome, plasmid, and
integrons . P. aeruginosa, K. pneumoniae, and A. baumannii produce class B
enzymes encoded by mobile genetic elements. In contrast, Bacillus spp.,
Chryseobacterium spp., and Stenotrophomonas maltophilia possess
chromosomally encoded MBLs, but the majority of these pathogens are
generally not responsible for serious infections.
b-lactamase
Class C serine cephalosporinases
Class C AmpC b–lactamases include CMY-2, P99, ACT-1, and DHA-1,
which are usually encoded by chromosomal bla genes, although plasmidborne AmpC enzymes are becoming more prevalent. Organisms
expressing the AmpC b–lactamase are resistant to penicillins, b-lactam–blactamase inhibitor combinations, and cephalosporins, including cefoxitin,
cefotetan, ceftriaxone, and cefotaxime. AmpC enzymes poorly hydrolyze
cefepime and are inhibited by cloxacillin, oxacillin, and aztreonam.
Members of the Enterobacteriaceae family, such as Enterobacter spp. and
Citrobacter spp., are AmpC b-lactamase producers that resist inhibition by
clavulanate and sulbactam, although Klebsiella spp., Salmonella spp., and
Proteus spp. normally do not harbor chromosomal blaAmpC genes.
Production of chromosomal AmpCs in Gram-negative bacteria is at a low
level (“repressed”) but can be “derepressed” by induction with certain blactams, particularly cefoxitin.
b-lactamase
Class D serine oxacillinases
Class D b-lactamases were initially categorized as “oxacillinases” because
of their ability to hydrolyze oxacillin at a rate of at least 50% of that of
benzylpenicillin, in contrast to the relatively slow hydrolysis of oxacillin by
classes A and C. In bacteria, OXA b–lactamases can also confer resistance
to penicillins, cephalosporins, extended-spectrum cephalosporins (OXAtype ESBLs), and carbapenems (OXA-type carbapenemases). OXA
enzymes are resistant to inhibition by clavulanate, sulbactam, and
tazobactam, (with some exceptions; e.g., OXA-2 and OXA-32 are
inhibited by tazobactam but not sulbactam and clavulanate, and OXA-53
is inhibited by clavulanate). Examples of OXA enzymes include those
rapidly emerging in A. baumannii (e.g., OXA-23 and OXA-24/40) and
constitutively expressed in P. aeruginosa (e.g., OXA-50).
-
Inibitori delle β-lattamasi
Alcuni β-lattamici, derivati dalla ricerca di nuovi
antimicrobici, sono stati inizialmente scartati a
causa della loro modesta potenza antibatterica nei
confronti dei vari patogeni. Un numero ristretto di
queste molecole è stato in seguito identificato
come capace di legarsi in modo covalente alle βlattamasi. Attraverso tale meccanismo l’enzima
viene catturato e reso indisponibile per inattivare
l’altro eventuale farmaco presente.
Inibitori delle β-lattamasi
Gli inibitori suicidi oggi in uso sono:
acido clavulanico, trovato in colture di
Streptomyces clavurigerus, tazobactam
che è un sulfone dell’acido penicillanico
e sulbactam che è un 6desaminopenicillino sulfone. Questi
composti sono abbinati a penicilline o
cefalosporine garantendo loro immunità
dalle più diffuse β-lattamasi.
Currently available b-lactamase inhibitors are effectively limited to many
class A b-lactamases, excluding the KPC carbapenemases.
Ceftolozane--tazobactam
CXA-101 (oxyimino-aminothiazolyl cephalosporin) + tazobactam (beta-lactamase
inhibitor) (2:1 ratio)
Microbiology Coverage •P. aeruginosa (MIC90 = 1-8 μg/ml)
•CXA-101 is not a substrate of the MexAB-OprM, MexCD-OprJ, MexEF-OprN, and MexXY
efflux pumps nor the carbapenem-specific porin OprD in P. aeruginosa
Spectrum Gaps •Bacteria producing metallo-lactamases and certain ESBLs (OXA-15 and OXA11, -14 and 16) confer losses of CXA-201 activity (MIC >32 μg/ml); derepression of ampC in P.
aeruginosa can increase MIC up to 8-fold; MIC90 vs. MDR P. aeruginosa of >64 μg/ml
•Ceftazidime-R E. coli, K. pneumoniae ceftazidime-R or KPC producers, Enterobacter, Citrobacter,
ESBL+ Proteus spp. (MIC90= 16/>16/>16/>16/>16 μg/ml)
•No activity against MRSA and other key gram-positives
IV only
The addition of tazobactam at 8 μg/ml resulted in restoring the susceptibility of 93% of the
ESBL producers and 95% of the AmpC overproducers. Tazobactam was unable to lower
MICs, however, for Enterobacteriaceae producing KPC carbapenemases
Molecole in evoluzione
Tra gli inibitori suicidi che non includono i
beta-lattamici vi sono acido boronico,
fosfonati (poco stabili e sensibili alle
fosfodiesterasi) e diazabiciclo octanoni
Avibactam inibisce B, C e D betalactamasi, ma non le metallo betalattamasi.
E’ in fase di sperimentazione clinica
Avibactam
DBO
diazabiciclo octanoni
Avibactam has an extremely broad
spectrum of activity against classes A
and C serine b-lactamases, including
ESBLs and class A carbapenemases
This molecule inhibits selected class
D b-lactamases including OXA-48,
but apparently not other class D
carbapenemases, as judged by the
absence of synergy with imipenem
against resistant strains of A.
baumannii producing OXA-51 and
OXA-58 and does not inhibit class B
MBLs.
Ceftazidime-avibactam
Microbiology •Covers ESBL-producing
Enterobacteriaceae and P. aeruginosa isolates
producing class A and C β-lactamases,
including KPC producers as well as AmpCoverexpressing strains
Spectrum Gaps •Not active against bacteria
producing metallo-lactamases, OXA or VEB
ESBLs, OXA carbapenemases in A. baumannii,
NDM-1 producers, and efflux-mediated
ceftazidime resistance in P. aeruginosa
IV only
Ceftaroline - avibactam
•Gram-positives: MRSA, VRSA, MRSE, VRE ( E. faecalis, ampsensitive), S. pneumoniae, S. pyogenes
•Gram-negatives: H. influenzae, M. catarrhalis, ESBL-producing
Enterobacteriaceae, wild type Acinetobacter, KPC-producers
•Atypicals: No coverage
•Anaerobes: No, requires combination with metronidazole Spectrum
Gaps: Acinetobacter producing OXA β-lactamases ,
Enterobacteriaceae producing metallo-β-lactamases, P. aeruginosa
producing AmpC or with reduced outer membrane permeability,
amp-resistant E. faecalis
Mutant Selection: Frequencies for stable mutants from 25
enterobacteria with ESBL, AmpC or KPC β-lactamases were mostly
< 10-9
IV only
Aztreonam-Avibactam
Monobactams are
hydrolyzed by ESBLs
but are inherently
stable to hydrolysis by
MBLs, an avibactamaztreonam
combination resulted
inhibitory to MBLproducing pathogens.
MK 7655
MK-7655, a new member of the
DBO series. A combination of
imipenem and MK-7655 has
excellent activity against a KPC2-producing isolate of K.
pneumoniae
and
displays
moderate improvements in the
imipenem efficacy against most
AmpC-overexpressing isolates
of P. aeruginosa.
Imipenem-MK7655
Spectrum with Imipenem
•Gram-positives: streptococci, MSSA, MSSE, non-VRE
•Gram-negatives: At 4-8 μg/ml MK-7655, imipenem MICs of all strains were
below the resistant breakpoint of IPM for Pseudomonas and Klebsiella
containing KPCs
•Atypicals: No coverage
•Anaerobes: Coverage of B. fragilis and others Spectrum Gaps: VRE, MRSA,
Stenotrophomonas spp., Burkholderia spp. strains containing class D
metalloproteases; high levels of AmpC or KPC; certain class A βlactamases Mutant Selection: 10-8 - 10-9 for 2 isolates of P. aeruginosa; 2
x10-7 to <3 x 10-8 for KPC+ K. pneumoniae for imipenem + MK-7655
IV only
Preclinical Findings
•Restored activity of imipenem to kill rapidly
•Low protein binding (20%)
RPX7009
RPX7009, a boronic acid-containing lactamase inactivator with inhibitory
activity against class A and class C
serine -lactamases, particularly
highlighting both in vitro and in vivo
activity against KPC-producing K.
pneumoniae. Boronic acid inhibitors
of PBPs and of classes A, C, and D lactamases had previously been
identified, but none has achieved
success as a clinical candidate.
Preclinical testing of RPX7009
indicated that the inhibitor had no
off-target effects, and it was well
tolerated at high doses, with no safety
signals that would preclude future
development
Biapenem-RPX7009
Biapenem (“RPX- 2003”) is a broad-spectrum
carbapenem with in vitro activity against Gram-negative
and Gram-positive bacteria similar to that of meropenem.
Like other carbapenems, biapenem is not affected by the
presence of ESBLs but is labile to hydrolysis by both
serine and metallo-carbapenemases. Although
carbapenem resistance is increasing, 75% of recent
Japanese pseudomonal isolates were susceptible to
biapenem and meropenem. Pharmacologically, biapenem
is notable for its low proconvulsive activity compared to
that of imipenem
Carbapenem + carbapenem
A combination of ertapenem and doripenem in both an in vitro chemostat
and an in vivo murine thigh infection model. Overall, the combination of
doripenem plus ertapenem demonstrated enhanced efficacy over either
agent alone (2011)
Double-Carbapenem Therapy Not Proven To Be More Active than
Carbapenem Monotherapy against KPC-Positive Klebsiella pneumoniae
(2012)
Ertapenem plus doripenem or meropenem were given in three patients
suffering from pandrug-resistant, KPC-2-positive Klebsiella pneumoniae
bacteremia (2 patients) and urinary tract infection (1 patient), respectively.
All responded successfully, without relapse at follow-up. The results
obtained should probably be attributed to ertapenem’s increased affinity
for the carbapenemases hindering doripenem/meropenem degradation in
the environment of the microorganism (2013)
Enzimi autolitici
La lisi cellulare ottenuta con le penicilline non è
dovuta alla semplice inibizione della sintesi del
peptidoglicano ma all’induzione di enzimi:
un’amilasi che scinde i legami tra i tetrapeptidi e,
una glucosilasi che scinde il glucano.
Quando questi enzimi sono inattivati o per
mutazione o per crescita dei batteri a basso pH, la
penicillina ha effetti batteriostatici.