A 43 year old woman with a history of metastatic breast cancer treated with radiation and chemotherapy presents to the emergency department with complaints of productive cough, blood tinged sputum, stabbing chest pain that is made worse by cough and deep inspiration, fevers and body aches for ten days. Her vitals are significant for temperature of 102 F, BP 95/50, RR 25, HR 130. A CT PE is obtained which is negative for pulmonary embolism with consolidation in the right lower lung field. You suspect a diagnosis of community acquired pneumonia. Upon chart review, you see she has the following drug allergy: anaphylaxis to penicillins. What is the most appropriate antibiotic regimen for this patient?

A. Ceftriaxone and Azithromycin 

B. Aztreonam and Azithromycin 

C. Azithromycin 

D. Doxycycline

Answer: B

Using the pneumonia severity index (below) this patient with a history of malignancy and vitals signs as above has a score of >70 and would benefit from a course of inpatient antibiotics and treatment. Inpatient antibiotic selection for CAP consists of a cephalosporin antibiotic + a macrolide or a fluoroquinolone alone. However, this patient has a history of anaphylaxis to penicillin antibiotics. Penicillins have a 10% cross reactivity with cephalosporin antibiotics, so a cephalosporin antibiotic would not be appropriate for this patient with a severe penicillin allergy. Therefore, aztreonam and azithromycin would be the preferred antibiotic regimen. Azithromycin has both gram-positive and gram-negative coverage. Azteonem offers gram-negative coverage and covers against pseudomonas; it is the preferred antibiotic for patients with a history of anaphylaxis to penicillins. Aztreonem’s MOA: inhibits bacterial cell wall synthesis by binding to one or more of the penicillin-binding proteins (PBPs) which in turn inhibits the final transpeptidation step of peptidoglycan synthesis in bacterial cell walls, thus inhibiting cell wall biosynthesis. Bacteria eventually lyse due to ongoing activity of cell wall autolytic enzymes (autolysins and murein hydrolases) while cell wall assembly is arrested. Monobactam structure makes cross-allergenicity with beta-lactams unlikely.

References: 

Lutfiyya MN, Henley E, Chang LF, Reyburn SW. Diagnosis and treatment of community-acquired pneumonia. Am Fam Physician. 2006 Feb 1;73(3):442-50. PMID: 16477891.

Azactam (aztreonam) [prescribing information]. Princeton, NJ: Bristol-Myers Squibb Company; received April 2021.

File TM. Treatment of community-acquired pneumonia in adults who require hospitalization. Post TW, ed. UpToDate. Waltham, MA: UpToDate Inc. http://www.uptodate.com. Accessed September 13, 2021.