Oral cephalosporin and β-lactamase inhibitor combinations for ESBL-producing Enterobacteriaceae urinary tract infections

Abstract

ESBL-producing Enterobacteriaceae as uropathogens have given rise to a sizeable amount of global morbidity. Community and hospital surveillance studies continue to report increasing proportions of these organisms as causes of urinary tract infection (UTI). Due to limited treatment options and the presence of cross-resistance amongst oral antibiotics of different classes, patients often require IV therapy, thereby increasing healthcare costs and reducing the effectiveness of delivering healthcare. Oral cephalosporin antibiotics are well known for their ability to achieve high urinary concentrations, in addition to achieving clinical success for treatment of uncomplicated UTI with a drug-susceptible pathogen. Novel cephalosporin/β-lactamase inhibitor combinations have been developed and demonstrate good in vitro activity against ESBL-producing isolates. A pooled analysis of in vitro activity of existing oral cephalosporin/clavulanate combinations in ESBL-producing Enterobacteriaceae has shown MIC₅₀s of 0.5–1, 0.125–1 and 0.25 mg/L for cefpodoxime, ceftibuten and cefixime, respectively. A novel cyclic boronic acid β-lactamase inhibitor, QPX7728, was able to produce MIC₅₀ values of 0.5 and ≤0.06 mg/L when paired with cefpodoxime and ceftibuten, respectively. Other novel combinations, cefpodoxime/ETX0282 and ceftibuten/VNRX7145, have also demonstrated excellent activity against ESBL producers. Clinical trials are now awaited.

Introduction

Antimicrobial resistance continues to be a major threat to the delivery of effective healthcare worldwide. MDR Gram-negative infections are now commonly seen in a variety of infections and contribute substantially to this global public health problem.¹ In the USA, of the 140 000 healthcare-associated Enterobacteriaceae infections per year, 26 000 (18.6%) are MDR (predominantly due to EBSL-producing organisms), resulting in close to 1700 deaths.² Likewise, a point prevalence survey of US hospitals in 2017 revealed over 290 000 non-duplicate ESBL-producing Enterobacteriaceae isolates.³ Gram-negative uropathogens causing urinary tract infection (UTI) are a major contributor to global antibiotic use and resistance.⁴ Treatment options for ESBL-producing uropathogens are often limited compared with non-ESBL-producing pathogens. IV antimicrobial therapy is frequently unavoidable as coexisting resistance to other oral agents (e.g. fluoroquinolones and sulphonamides) is often seen among ESBL producers. Nitrofurantoin and fosfomycin have provided some benefit in this patient group in the setting of uncomplicated UTI.⁵ They do not exhibit cross-class resistance with other classes of antibiotics and their usage increased significantly in the late 2000s, associated with the reported rise in ESBL-producing bacteria. As such, they have now become first-line agents for uncomplicated UTI in several international guidelines.⁶ This now requires monitoring due to an observed decline in efficacy due to their inability to adequately treat complicated UTI and the slow development of resistance owing to their increased use.⁷ In addition, fosfomycin activity against Klebsiella spp. has recently been challenged in an in vitro dynamic bladder model.⁸ The older extended-spectrum penicillins, pivmecillinam (also known as pivoxil amdinocillin) and temocillin, have also demonstrated good in vitro and in vivo activity against EBSL producers causing UTI.^5,9–12 Pivmecillinam has significant antibacterial potency, is stable to β-lactamase hydrolysis and shows synergy when paired with β-lactamase inhibitors against ESBL-producing Enterobacteriaceae.^13,14 A recent cohort study showed that oral pivmecillinam 400 mg three times daily yielded equivalent clinical outcomes in UTI caused by ESBL producers when compared with alternative oral antibiotics.¹⁰ More robust data from a randomized controlled trial are necessary in order for this strategy to be adopted worldwide. Recently, new compounds pairing orally bioavailable cephalosporin antibiotics with novel β-lactamase inhibitors have been developed. This promises the potential to improve oral treatment options and reduce healthcare costs in patients with UTIs due to ESBL-producing organisms.

Epidemiology of ESBL-producing organisms causing UTI

ESBLs render penicillins (except mecillinam and temocillin), first-, second- and third-generation cephalosporins and monobactams ineffective via hydrolysis of the β-lactam ring.^5,15 Uropathogens harbouring ESBLs, for example Escherichia coli ST131, are often resistant to other commonly used antibiotics, for example fluoroquinolones via point mutations in the gyrA and parC genes.^16,17 The presence of ESBL genes among Enterobacteriaceae isolates has been progressively increasing in both hospital and community settings, representing 15% of all Klebsiella spp. isolates and 12% of E. coli captured among 79 US hospitals.¹⁸ Considerable global variability exists between countries, however, with Europe exhibiting 6.8%–38.7% of E. coli and 4.5%–77.7% of Klebsiella pneumoniae as resistant to third-generation cephalosporins.¹⁹ Similarly, the incidence of community-acquired ESBL-producing Enterobacteriaceae as a cause of UTI has increased worldwide, with some areas reporting rates of over 40% of all UTIs.²⁰ Case–control studies have identified numerous classical risk factors for acquisition of ESBL-producing Enterobacteriaceae, which include recent antibiotic use (in particular fluoroquinolones), hospital admission and surgery.²¹ The urinary tract is the most common site of infection caused by ESBL producers and results in a sizeable economic and public health burden.²² The hospitalization cost of an ESBL-producing E. coli UTI compared with a non-ESBL-producing E. coli was estimated to be €4980 versus €2612.²³

The most common globally disseminated ESBL genes found amongst uropathogenic Enterobacteriaceae are those of the CTX-M group.⁴ In the modern era of increasing rates of MDR Enterobacteriaceae among urinary tract pathogens, oral agents are likely to be less active compared with the time when many guidelines were written. Unless novel oral antimicrobials are developed, the requirement for IV therapy will contribute significantly to the burden of care administered in hospitals and will no doubt drive a large increase in healthcare costs.

Oral cephalosporins (without β-lactamase inhibitors) and their activity against Enterobacteriaceae

Orally bioavailable cephalosporins are represented in first-, second- and third-generation classes. All have some level of in vitro activity against non-ESBL-producing Enterobacteriaceae commonly isolated from urine. EUCAST has specified clinical breakpoints for uncomplicated UTI against Enterobacteriaceae (Table 1) for cefadroxil, cefuroxime, cefalexin, cefixime, cefpodoxime and ceftibuten (susceptible if MICs are ≤16 mg/L, ≤8 mg/L, ≤16 mg/L, ≤1 mg/L, ≤1 mg/L and ≤1 mg/L respectively). However, CLSI has recommended utilizing the cefazolin breakpoint for Enterobacteriaceae due to uncomplicated UTI (susceptible ≤16 mg/L) as a surrogate for oral agents (cefaclor, cefdinir, cefpodoxime, cefprozil, cefuroxime, cefalexin and loracarbef) and where tested as non-susceptible to cefazolin, laboratories may go on to test cefdinir, cefpodoxime and cefuroxime due to resistance over-calling by cefazolin.²⁴

Table 1.

Oral cephalosporin EUCAST and CLSI breakpoints for UTI

Cephalosporin	EUCAST susceptible breakpointa (mg/L)	CLSI susceptible breakpointa,b (mg/L)
First generation
cefadroxil	≤16	NA
cefalexin	≤16	NA
Second generation
cefuroxime	≤8	≤4
cefaclor	NA	≤8
cefprozil	NA	≤8
loracarbef	NA	≤8
Third generation
cefpodoxime	≤1	≤2
cefixime	≤1	≤1
cefdinir	NA	≤1
ceftibuten	≤1	≤8
cefetamet	NA	≤4

NA, data not available.

a

Uncomplicated UTI only.

b

Cefazolin may be used as a surrogate (susceptible: ≤16 mg/L).

First-generation cephalosporins

Cefalexin has activity against most Enterobacteriaceae causing UTIs; however, this activity is significantly reduced against ESBL producers. In one study of non-ESBL-producing uropathogens, overall susceptibility was 85% (Table 2). However, susceptibility of 981 ESBL-producing E. coli and 439 ESBL-producing K. pneumoniae urinary isolates was only 6.32% and 0.91%, respectively, in one large study.²⁵ Cefadroxil, another first-generation cephalosporin, revealed average potency overall and a broad MIC₅₀ range of 4 to >32 mg/L in a pooled analysis of 164 isolates.^26,27

Table 2.

Pooled WT MIC data of oral cephalosporins against Enterobacteriaceae for which UTI breakpoints exist

Oral cephalosporin	Reference(s)	Total no. of isolates tested across all studies	MIC50 (mg/L)	MIC90 (mg/L)	Range (mg/L)	Susceptibility (%)
Cefalexin	94,95	126	4–8	8–32	1 to >256	54.5–85
Cefadroxil	26,27	164	8–16	4 to >32	4 to >32	—
Cefprozil	94,95	126	2	4–8	0.5 to >16	90–93.2
Cefaclor	42,95,96	1432	2–16	4 to >32	≤0.5 to >128	47.5–90.5
Cefuroxime	27,42,94,95	837	2–8	4 to >32	0.25 to >32	50.5–86.4
Cefdinir	94,95	126	0.12–0.5	0.5–1	0.12 to >4	90–95.2
Cefixime	42,97	969	≤0.06–0.5	0.5 to >8	0.06 to >16	58.2–81.3
Cefpodoxime	42,95,97	1011	0.12–1	0.5 to >8	0.06 to >16	53.5–95.2
Ceftibuten	27,42	711	0.12–0.25	0.5 to >16	0.01 to >16	66.3–91.3

Second-generation cephalosporins

Second-generation oral cephalosporins such as cefaclor exhibit greater activity against WT Gram-negative bacilli than does cefalexin. Although still displaying a broad range of susceptibility results—the majority reported against EUCAST breakpoints—cefaclor (47.5%–90.5%), cefuroxime (50.5%–86.4%) and cefprozil (90%–93.2%) were active against the majority of uropathogens (Table 2). As with first-generation cephalosporins, however, these antibiotics have poor susceptibility profiles against ESBL producers.²⁸

Third-generation cephalosporins

Oral third-generation cephalosporins perform better still, with MICs among Enterobacteriaceae consistently below EUCAST/CLSI breakpoints for UTI. They demonstrate good in vitro efficacy against uropathogenic Enterobacteriaceae and also exhibit some resistance to hydrolysis by common ESBLs (Table 3). Cefpodoxime is a cephalosporin with activity against Gram-negative bacteria and shows stability to many β-lactamases.^29,30 It exists as a prodrug, cefpodoxime proxetil, that undergoes de-esterification in the intestinal wall before active cefpodoxime (typically 50%) is released into the circulation.³¹ Optimal absorption requires low gastric pH and a mean peak serum concentration of 2.18 g/L can be expected 3 h post administration of a 200 mg dose.³² A dose of 100 mg 12 hourly has been recommended for uncomplicated UTI.³³ Similarly, ceftibuten is another third-generation cephalosporin with stability to plasmid-mediated β-lactamases.³⁴ Ceftibuten resists hydrolysis by commonly occurring narrow-spectrum β-lactamases (e.g. OXA-1, TEM-1 and SHV-1 types).³⁵ However, it is readily hydrolysed by SHV-4/5 produced by some Klebsiella isolates.³⁶ Ceftibuten is absorbed (75%–90%) in its active form from the gastrointestinal tract, achieving peak plasma antibiotic levels of 9.9 mg/L and 17 mg/L after single oral doses of 200 mg and 400 mg, respectively.³⁷ Drug clearance is split between renal (56%) and faecal (39%) excretion. Typical doses are 400 mg daily for respiratory tract infection; however, no dosing recommendations exist for UTI.³⁸

Table 3.

In vitro activity of oral cephalosporins with or without β-lactamase inhibitors against ESBL-producing Enterobacteriaceae

Oral cephalosporin ±  β-lactamase inhibitor	Reference(s)	Total no. of isolates tested across all studies	MIC50 (mg/L)	MIC90 (mg/L)	Range (mg/L)	Susceptibility (%)
Cefixime	78,81	214	6	>64	0.5 to >64	7–8.6
Cefpodoxime	85,87,98	386	>16 to >64	>16 to >64	0.5 to >64	0–1.2
Ceftibuten	12,85,98,99	1044	2–8	16 to >64	NA	1–56.9
Cefpodoxime/clavulanate	73,75–77,81	1144	0.5–1	0.5 to >32	≤0.06 to >32	58.8–75
Ceftibuten/clavulanate	91	4	0.125–1	0.125–1	NA	NA
Cefixime/clavulanate	78,79,81	276	0.25	0.75	0.09–24	86.3–90
Cefpodoxime/QPX7728	84,85	NA	0.5	4	NA	NA
Ceftibuten/QPX7728	84,85	NA	≤0.06	1	NA	NA
Cefpodoxime/ETX0282	87,100	937	≤0.015–0.5	0.03–1	0.12–2	NA
Ceftibuten/VNRX7145	88,89,99,101	884	0.06 to <1	0.12–1	NA	96.9–100

NA, data not available.

Among 301 US Enterobacteriaceae isolates, cefpodoxime demonstrated a higher susceptibility rate than cefuroxime (94% versus 83%).³⁹ Cefpodoxime and ceftibuten also performed well against 155 ESBL-negative MDR E. coli and K. pneumoniae urinary isolates, showing 94% susceptibility each for cefpodoxime and 100% each for ceftibuten.⁴⁰ Among 1190 Enterobacteriaceae isolates causing UTI in Europe, ceftibuten and cefpodoxime had susceptibility rates of 94% and 92%, respectively, with ceftibuten demonstrating in vitro efficacy similar to that of ceftriaxone.⁴¹ Furthermore, 567 community-acquired UTI Enterobacteriaceae isolates from Argentina, Mexico, Venezuela, Russia and the Philippines showed ceftibuten in vitro potency, being able to outperform other oral cephalosporins.⁴² The ceftibuten susceptibility rate was 77% by EUCAST, with MIC_50/90 of 0.25/16 mg/L and a range of 0.06 to >16 mg/L. Cefixime, cefpodoxime, cefuroxime and cefaclor had in vitro susceptibilities of 67%, 65%, 65% and 58%, respectively.⁴² Ceftibuten alone was tested in vitro against CTX-M-producing E. coli isolates and showed a disparity in susceptibility between CTX-M group 1 and group 9, with MIC_50/90 values of 8/32 and 1/1 mg/L, respectively, and susceptibility rates of 9.8% and 94.5%, respectively.¹²

Antibiotic concentrations in the urinary tract

First-generation cephalosporins

Oral cephalosporins have been shown to have favourable pharmacokinetics and to achieve high concentrations of active drug in the urine. Cefalexin is excreted unchanged in the urine, with 70%–100% of the dose found in the urine 6–8 h after a dose and concentrations many times greater than the MIC for a typical urinary pathogen (urine cefalexin concentration 500–1000 mg/L).⁴³ Although increasing resistance rates to cefalexin have been noted, these high urinary concentrations may allow it to retain effectiveness for uncomplicated UTIs.⁴⁴ Similarly, cefadroxil is excreted in urine (via glomerular filtration and tubular secretion), with 93% of the dose passing through the urine in the first 24 h, achieving concentrations between 400 and 2400 mg/L.²⁶ Its urinary excretion rate is more prolonged when compared with cefalexin.⁴⁵

Second-generation cephalosporins

Cefaclor achieves slightly lower urinary antibiotic concentrations overall (50–1000 mg/L), with 70% of active drug passing through the urine in the first 6 h.⁴⁶ It also has other major non-renal routes of elimination, such as the biliary tract.⁴⁷ Cefprozil shows slightly poorer urinary concentrations, excreting around 61% of the administered dose and achieving urinary concentrations of 175 mg/L and 658 mg/L at 4 h after a 250 mg and 1 g dose, respectively.⁴⁸ Cefuroxime axetil (oral formulation) has its parent compound, cefuroxime, eliminated unchanged in the urine (42%–57% in 24 h after a single oral 250 mg dose) and achieves high concentrations in the renal parenchyma.⁴⁹ It achieves peak concentrations of 400 mg/L at 4 h after a 500 mg oral dose.⁵⁰

Third-generation cephalosporins

Third-generation or extended-spectrum oral cephalosporins also penetrate the kidneys and urinary tract well. Cefpodoxime clearance occurs primarily through the renal route by both glomerular filtration and tubular secretion.³² Over various approved doses (100–400 mg), 29%–33% of administered cefpodoxime was excreted unchanged in urine in a 12 h period.⁵¹ This increased to 42.8% over 24 h after a 100 mg dose was given to healthy subjects.³² Urinary excretion is 80% of the absorbed oral dose (approximately 50% of total administered dose). After 8 days of 400 mg twice-daily oral dosing of cefpodoxime proxetil, 146 mg (35.6%) and 122 mg (30.6%) had been excreted in the urine in 12 h in young and elderly healthy adults, respectively.⁵² Ceftibuten demonstrates a fractional excretion of unchanged drug in the urine, with 56%–70% over 24 h. After an oral dose of 200 mg of ceftibuten, it can achieve a maximum urinary concentration of approximately 800 mg/L.⁵³ Cefixime undergoes less urinary clearance. After administering 200 mg, 53.3 mg (27%) can be recovered from the urine after 24 h.⁵⁴ In another study, administering 200 mg and 400 mg of cefixime yielded urinary recovery of 20% and 16% of the dose.⁵⁵ Maximum concentrations achieved in the urine were 107 mg/L and 164 mg/L for a 200 mg and 400 mg dose, respectively. Cefdinir has an even lower fractional elimination of unchanged drug in the urine (13%–23%).⁵⁶ In one study, the mean percentage of the dose recovered unchanged in the urine following 300 mg and 600 mg doses was 18.4% and 11.6%, respectively, over 24 h.⁵⁷ Although third-generation cephalosporins achieve lower urinary concentrations when compared with first-generation cephalosporins, both cefpodoxime and ceftibuten appear to be superior to cefixime and cefdinir.

Oral clavulanate

Clavulanate is usually paired with amoxicillin and has been used in the treatment of UTI. Clavulanate is also excreted in the urine in its active unchanged form, although far less than many oral cephalosporins. After administration of 125 mg of clavulanic acid, cumulative excretion at 2, 4 and 6 h was 14%, 25.7% and 27.8%, respectively.⁵⁸ Other studies have documented similar values of between 18% and 38% of total excretion. Clavulanate reaches a maximum urinary concentration of 40 mg/L, 4–6 h after a 125 mg oral dose. Approximately half of the drug is metabolized elsewhere in the body.⁵⁹

Clinical trials of orally administered cephalosporins (without β-lactamase inhibitors) for UTI

Numerous clinical trials have been conducted examining the use of oral cephalosporins in the treatment of UTI. The majority of patients enrolled in these trials had uncomplicated infection with urinary pathogens without ESBL production.

First-generation cephalosporins

Oral cefalexin has been utilized as a comparator arm in many therapeutic trials for uncomplicated UTI. In 109 patients receiving oral cefalexin 250 mg four times a day for acute uncomplicated UTI, 93.6% and 80.6% were able to achieve clinical cure and microbiological cure, respectively.⁶⁰ In a trial comparing cefalexin 500 mg with ampicillin 500 mg, each four times a day, 30/31 (96.8%) patients who received cefalexin achieved clinical cure and 20/31 (64.5%) had a sterile urine sample collected 3 weeks after commencement of therapy.⁶¹ Furthermore, in another double-blind clinical trial, of 115 patients with complicated UTI receiving oral cefalexin 500 mg four times a day, clinical cure was reported in only 75 (65.2%) patients.⁶² Cefadroxil 1 g twice daily was compared with norfloxacin 400 mg twice daily in the treatment of acute pyelonephritis in a multicentre clinical trial that enrolled 197 patients.⁶³ Although cefadroxil produced an inferior microbiological cure rate (65% versus 98%), the clinical response rate did not differ between groups. With regard to clinical cure, 7 days of oral cefadroxil 1 g daily performed just as well as amoxicillin 375 mg three times a day in women with uncomplicated UTI.⁶⁴

Second-generation cephalosporins

The second-generation cephalosporins cefprozil and cefaclor have been compared against each other in a number of clinical trials for uncomplicated UTI. In one study of cefprozil 500 mg daily and cefaclor 250 mg three times daily, clinical response was 87% versus 84% and microbiological cure was 83% versus 85%.⁶⁵ Among 108 college women with acute UTI, the clinical response was 94% versus 94% and microbiological cure was 93% versus 94%, respectively.⁶⁶ Oral cefuroxime axetil 125 mg twice daily was compared with enoxacin (a fluoroquinolone) in a small clinical trial involving 33 patients with recurrent UTI and achieved clinical response and microbiological cure rates of 71.4% and 71.4%, compared with 92.3% and 85.7% in the enoxacin group.⁶⁷ In a large clinical trial involving 672 women with uncomplicated UTI, treated with cefuroxime axetil 125 mg twice daily or 250 mg twice daily, 628/672 (93.4%) achieved clinical cure.⁶⁸

Third-generation cephalosporins

Cefdinir 100 mg twice daily was compared with cefaclor 250 mg three times a day, both for 5 days in uncomplicated UTI, in a large multicentre trial involving 661 participants.⁶⁹ Cefdinir achieved a clinical response and microbiological cure rate of 85.9% and 91.3%, respectively, with cefaclor achieving similar results (80.5% and 93%, respectively). With regard to extended-spectrum oral cephalosporins, cefpodoxime 100 mg twice daily did not meet the criteria for non-inferiority when compared with oral ciprofloxacin in a randomized trial in acute uncomplicated cystitis.⁷⁰ Patients who received oral cefpodoxime proxetil 100 mg twice daily for 3 days achieved an overall clinical cure rate at 30 day follow-up in the ITT population of 82% (123/150) compared with 93% (139/150) in those receiving oral ciprofloxacin 250 mg twice daily.⁷⁰ The authors of that study speculate that the lower clinical response seen with β-lactam therapy may be due to their poorer activity in eradicating uropathogens from the vaginal flora.⁷⁰ Cefpodoxime 100 mg twice daily was also compared with trimethoprim/sulfamethoxazole in acute uncomplicated cystitis in a small open-label multicentre trial, which showed that 3 days of cefpodoxime was as effective as trimethoprim/sulfamethoxazole.⁷¹ Another small randomized clinical trial compared ceftibuten 200 mg twice daily and cefixime 200 mg twice daily in the treatment of complicated UTI and showed similar clinical and microbiological outcomes between the two drugs.⁷² Thus, it is unclear from clinical trial data which oral third-generation cephalosporin is superior with regard to clinical and microbiological cure. It should also be pointed out that all of these studies were performed in scenarios where ESBL-producing strains were rarely encountered.

Oral cephalosporin and clavulanic acid combinations

Clavulanic acid is a highly effective inhibitor of ESBLs in vitro and is typically combined with amoxicillin in both oral and IV formulations.⁷³ Compared with other β-lactam/β-lactamase (BLBLI) combinations, amoxicillin is less stable to ESBLs even in the presence of clavulanate. Oxyimino-cephalosporins are weaker substrates than amoxicillin for ESBLs and have a higher affinity for PBPs.⁷³ A fixed-dose combination of cefuroxime/clavulanic acid has been approved and introduced in some countries, although currently this is not approved by the US FDA.⁷⁴ Many oral cephalosporin antibiotics have been tested in vitro, in combination with clavulanic acid, against ESBL-producing Enterobacteriaceae (Table 3). Among 380 ESBL-producing E. coli and 226 Klebsiella spp. isolates, cefpodoxime combined with clavulanate produced MIC_50/90 values of 0.5/2 and 0.5/1 mg/L, respectively.⁷³ Twenty-eight of 37 (75.6%) ESBL-producing E. coli from India had MIC values less than the cefpodoxime-susceptible breakpoint when combined with clavulanate.⁷⁵ In another in vitro study, 101 ESBL-producing Enterobacteriaceae were tested against cefdinir and cefpodoxime, in combination with amoxicillin/clavulanate at fixed concentrations of 8 mg/L and 4 mg/L respectively.⁷⁶ Utilizing CLSI cefdinir and cefpodoxime breakpoints prior to 2011, susceptibility amongst CTX-M producers was reported at 90.9% and 89.3%, respectively, whilst producers of SHV-type ESBLs were reported as susceptible for 60% and 58.8%, possibly due to differences in substrate affinity for CTX-M enzymes and SHV ESBL enzymes towards cefdinir and cefpodoxime (Table 4). In addition, against 303 CTX-M-producing E. coli, cefpodoxime/clavulanate demonstrated potent activity (MIC_50/90 0.5/0.5 mg/L) but, not surprisingly, was less active against those hyperproducing AmpC (MIC_50/90 4/4 mg/L).⁷⁷ Cefixime plus amoxicillin/clavulanate tested against 64 ESBL-producing E. coli isolates produced MIC_50/90 values of 0.25/75 mg/L (range 0.09–24 mg/L).⁷⁸ Cefixime/clavulanate also performed well in vitro against non-AmpC ESBL producers in 62 E. coli and K. pneumoniae isolates.⁷⁹ Both of these studies, however, demonstrated synergy using non-standardized methods not recommended by either EUCAST or CLSI, thereby limiting the value of their interpretation. Cefdinir plus a fixed concentration of amoxicillin/clavulanate (8/4 mg/L) was tested against CTX-M (46 isolates) and SHV/TEM (11 isolates) producers, which revealed in vitro MIC_50/90 values of 0.25/2 mg/L (89.1% cefdinir susceptible) and 0.06/8 mg/L (81.8% cefdinir susceptible), respectively.⁸⁰

Table 4.

Typical MICs (mg/L) of orally administered third-generation cephalosporins for selected β-lactamase-producing Enterobacteriaceae

		MIC (mg/L)
Organism and β-lactamase	References	cefdinir	cefixime	ceftibuten	cefpodoxime	MIC reduction with clavulanate?
E. coli + AmpC	102,103	>64	>64	>256	16 to >64	no
E. coli + CTX-M-1	12,36,82,103	8	4–16	1–8	1–64	yes
E. coli + CTX-M-9	12,82			1	0.5–1	yes
E. coli + TEM-1	30,102–106	0.5	0.25–0.5	0.12–1	0.5–1	yes
E. coli + TEM-2	103,105,106	0.12	0.06–0.12	0.12		yes
E. coli + OXA-1	30,105,106	0.5	0.1–0.25	0.25	0.5	yes
E. coli + OXA-2	102,105,106	0.25	0.25–0.5	0.5	0.5	yes
K. pneumoniae + CTX-M-1	36	NA	8	0.25	32	yes
K. pneumoniae + AmpC	36	NA	>64	>64	>64	no
K. pneumoniae + SHV-1	30	NA	NA	NA	0.25	yes
K. pneumoniae + K14	102,105,106	4	0.25	≤0.06	8	yes
K. pneumoniae + hyperexpressed K1	30,102,105,106	4	0.25	≤0.06	0.25–8	yes
Enterobacter + basal AmpC	30,35,102,106,107	0.12 to >128	4 to >32	0.5 to >32	2 to >128	no

NA, data not available.

The addition of amoxicillin/clavulanate to oral cephalosporins was shown to have high rates of synergy when tested using disc approximation and disc replacement methods amongst 150 ESBL-producing E. coli isolates predominantly isolated from urinary sources.⁸¹ The frequency of synergy using this method with cefixime and cefpodoxime was 84% and 75.5%, respectively. Moreover, 86.3% and 74.1% of isolates were in the susceptible range when cefixime and cefpodoxime were added to amoxicillin/clavulanate and interpreted using CLSI disc diffusion breakpoints. Twenty patients with confirmed ESBL-producing E. coli UTI, 85% of whom had demonstrable positive in vitro synergy, received cefixime and amoxicillin/clavulanate, with a 90% clinical cure rate observed. A case series of 10 patients treated with ceftibuten plus amoxicillin/clavulanate for ESBL-producing E. coli and K. pneumoniae UTI showed all patients achieving clinical cure.⁸² Overall, the addition of clavulanic acid to a third-generation cephalosporin was able to significantly reduce the MIC_50/90 against organisms producing specific ESBLs (CTX-M, TEM or SHV-1) with little activity against AmpC producers (Table 4).

Limitations of in vitro oral cephalosporin/β-lactamase inhibitor susceptibility data interpretation

In the current literature, cephalosporin/β-lactamase inhibitor susceptibility testing (as with other BLBLI combinations) appears to be frequently performed by non-standardized methods and exhibits great heterogeneity in methodology. Methods to determine antimicrobial susceptibility and synergy in oral cephalosporin/β-lactamase inhibitor combinations have included disc diffusion, broth microdilution (BMD), Etest, disc approximation and disc replacement.^73,75–77 In addition, different concentrations of β-lactamase inhibitors are often used amongst testing strategies. With regard to clavulanic acid, either fixed concentrations of 2 mg/L or 4 mg/L have been used, or ratios of 2:1 (cephalosporin:clavulanate) have been reported. Despite having standardized and accepted clinical breakpoints for both EUCAST and CLSI for many oral cephalosporins, when assessing them in combination with clavulanate, interpretation can be problematic as the two differ with regard to methodology (fixed 2 mg/L for EUCAST and 2:1 ratio for CLSI).⁸³ Many reported studies used fixed combinations of 4 mg/L, not complying with either EUCAST or CLSI. The spectrum of testing performed highlights a need for further research towards a unified approach. Ultimately, BMD and disc diffusion MIC distributions are required to determine appropriate breakpoints but, similarly to other BLBLIs currently available, it would seem prudent to assess activity for cephalosporin/clavulanate combinations based on whether the MICs determined by BMD lie within the susceptible distribution range for cephalosporins alone.

Novel β-lactamase inhibitors in combination with ceftibuten or cefpodoxime

There has been a recent drive to develop new oral antimicrobials that are active against ESBL-producing uropathogens. QPX7728 is a next-generation cyclic boronic acid β-lactamase inhibitor that is orally bioavailable and inhibits all clinically important β-lactamases.⁸⁴ It has been shown to restore the activity of cefpodoxime and ceftibuten on ESBL producers, with MIC_50/90 values of 0.5/4 and ≤0.06/1 mg/L, respectively.⁸⁵ ETX0282 is an oral prodrug that is hydrolysed in vivo to release ETX1317, another novel β-lactamase inhibitor that is orally bioavailable and inhibits Class A and C β-lactamases.⁸⁶ When paired with cefpodoxime it was able to significantly reduce bacterial titres in kidney, bladder and urine in a murine ESBL-producing E. coli UTI model.⁸⁶ ETX1317 was able to restore the activity of cefpodoxime in 1875 global Enterobacteriaceae UTI isolates, with MIC_50/90 values of 0.06/0.12 mg/L.⁸⁷ It demonstrated similar potency against AmpC producers. VNRX7145 is another β-lactamase inhibitor that undergoes biotransformation to the active VNRX5236 with potent activity against Class A, C and D enzymes, but not Class B. Among 100 isolates of Enterobacteriaceae, ceftibuten/VNRX5236 maintained an MIC₉₀ of ≤1 mg/L across all enzyme subgroups tested (ESBL, KPC, AmpC and OXA-48).⁸⁸ Moreover, VNRX5236 was able to reduce the MIC values of ceftibuten for a similar profile of susceptible isolates compared with ceftazidime/avibactam and meropenem against ESBL producers.⁸⁸ Of 50 ESBL-producing isolates tested, MIC_50/90 values were 0.06/0.12 mg/L, with 98% susceptible by EUCAST criteria for ceftibuten.⁸⁹ Ceftibuten/VNRX5236 has also demonstrated in vivo efficacy in a neutropenic thigh infection model against serine β-lactamase-producing Enterobacteriaceae, achieving a mean reduction in bacterial burden of −0.2 log₁₀ cfu/thigh.⁹⁰ Ceftibuten/clavulanate is another novel BLBLI combination that also has potential utility in the management of UTIs caused by ESBL producers. It exhibited promising activity in four ESBL-producing isolates.⁹¹ It has also shown favourable in vivo activity against ESBL-producing Enterobacteriaceae in murine thigh model studies and human clinical trials are planned.⁹² Pharmacodynamic studies have been helpful in designing potential dosage regimens for this combination molecule.⁹¹

A Phase 1 clinical trial to establish the safety and pharmacokinetics of ETX0282 is registered and underway.⁹³ Clinical trials involving these other new agents are expected to be registered in the near future.

Conclusions

The increase in frequency of ESBL-producing Enterobacteriaceae as causes of UTI worldwide is a concern. More alarming still is the limited number of oral antibiotic options currently available to clinicians to treat these infections. Recent enthusiasm and drug development directed towards cephalosporin/β-lactamase inhibitor combinations with good oral bioavailability has provided some much-needed optimism. Novel oral β-lactamase inhibitor compounds QPX7728, ETX0282 and VNRX5236, when paired with the oral third-generation cephalosporins ceftibuten and cefpodoxime were able to demonstrate good in vitro activity against MDR Gram-negative uropathogens. However, analysis of their potency is significantly limited by non-standardized testing methods and limited consensus regarding application of clinical breakpoints. Moreover, the combination of two established compounds, ceftibuten and clavulanate, has also demonstrated encouraging efficacy. A clinical trial assessing the safety and efficacy of cefpodoxime/ETX0282 is currently underway. Well-designed studies assessing the pharmacokinetic/pharmacodynamic profile of both new and old antimicrobials used to treat ESBL-producing UTI continue to be important. Orally administered carbapenems (e.g. tebipenem) are being developed and it remains to be seen how their efficacy compares with orally administered BLBLI combinations. Given the rising incidence of carbapenem-resistant organisms worldwide, the search to find a suitable ‘carbapenem-sparing’ agent has become a top priority.

Transparency declarations

P.N.A.H. has received funding from Pfizer, Merck Sharpe & Dohme (MSD), and Shionogi. D.L.P. has received funding from AstraZeneca, Leo Pharmaceuticals, Bayer, GlaxoSmithKline (GSK), Cubist, Venatorx and Accelerate; reports board membership from Entasis, Qpex, Merck, Shionogi, Achaogen, AstraZeneca, Leo Pharmaceuticals, Bayer, GSK, Cubist, Venatorx, and Accelerate; reports grants/grants pending from Shionogi and Merck; and has received payment for lectures including service on speaker’s bureaus from Pfizer, outside the submitted work. All other authors: none to declare.