INTRODUCTION: Sacituzumab govitecan-hziy, approved in 2020 for treatment of 
metastatic triple-negative breast cancer, provides a new option for a population 
with a historically poor prognosis with standard chemotherapy. Uridine 
diphosphate glucuronosyltransferase family 1 member A1 poor metabolizers are at 
increased risk for profound neutropenia. This case discusses clinical 
implications of the uridine diphosphate glucuronosyltransferase family 1 member 
A1*28/*28 genotype in patients receiving sacituzumab govitecan-hziy.
CASE REPORT: A 38-year-old otherwise healthy pre-menopausal female of South 
Asian descent was diagnosed with non-metastatic, hormone receptor-positive, and 
human epidermal growth factor receptor 2-negative breast cancer. This was 
treated with neoadjuvant chemotherapy and multiple lines of subsequent 
therapies. Upon finding bone metastasis, an additional six lines of therapy 
ensued. In total, 3.5 years post-diagnosis, sacituzumab govitecan-hziy was 
started for disease transformation to triple-negative status.
MANAGEMENT AND OUTCOME: Sacituzumab govitecan-hziy was initiated at the Food and 
Drug Administration-approved 10 mg/kg/dose on days 1 and 8 of a 21-day cycle. 
Grade 4 neutropenia occurred after one dose. Pharmacogenomics testing identified 
the patient as a uridine diphosphate glucuronosyltransferase family 1 member 
A1*28 homozygous expressor. Sacituzumab govitecan-hziy was dose-reduced, and 
granulocyte colony-stimulating factor was administered due to the severity of 
neutropenia. The patient continued on sacituzumab govitecan-hziy until disease 
progression.
DISCUSSION: Sacituzumab govitecan-hziy's propensity to cause neutropenia is 
multifactorial. Although incidence of all-grade neutropenia from sacituzumab 
govitecan-hziy is elevated for uridine diphosphate glucuronosyltransferase 
family 1 member A1*28 homozygous expressors, this does not translate to 
increased risk for febrile neutropenia. Detailed guidance is lacking regarding 
empiric dose adjustments or prophylactic granulocyte colony-stimulating factor 
for these patients.1 Currently, pre-sacituzumab govitecan-hziy pharmacogenomics 
testing to identify uridine diphosphate glucuronosyltransferase family 1 member 
A1 poor metabolizers is not recommended, and the cost-effectiveness of this 
approach is unclear.