Chemotherapy Drug Speeds Ovarian Aging by Breaking the Body’s Cellular Clock, Study Finds

Cyclophosphamide is one of the most widely prescribed chemotherapy drugs in the world, deployed against breast cancer, lymphomas, leukemias and a range of autoimmune conditions. Oncologists have long known that it carries a heavy price for young patients: the drug is notoriously toxic to the ovaries, capable of wiping out follicles and pushing women into premature ovarian insufficiency and infertility. For decades, the dominant explanation centered on DNA damage, the classic mechanism by which alkylating agents destroy dividing cells. But a new study published in the Journal of Ovarian Research suggests that this picture is incomplete, pointing instead to an unexpected chain of molecular events that links vitamin metabolism to the circadian clock deep inside the ovary.

The research, led by Yao Peng and Yong Tan of Nanjing University of Chinese Medicine together with colleagues, set out to map what actually happens inside ovarian tissue after repeated exposure to cyclophosphamide. Working with a rat model of multiple drug exposures, the team combined two powerful approaches: they measured the activity of circadian rhythm genes in the ovary, and they ran proteomic analysis to survey the full complement of proteins whose abundance shifted after treatment. The results converged on a surprising culprit. The ovaries of drug-exposed animals showed abnormal expression of circadian rhythm genes, and proteomics revealed a dramatic drop in one protein in particular: riboflavin kinase, abbreviated RFK, the enzyme that initiates the cellular processing of vitamin B2.

That finding might sound obscure, but riboflavin kinase sits at the head of a metabolic pathway with far-reaching consequences. The enzyme converts riboflavin, the vitamin better known as B2, into flavin mononucleotide, which is then elaborated into flavin adenine dinucleotide, or FAD, one of the most indispensable cofactors in biology. FAD participates in hundreds of redox reactions, shuttling electrons through the metabolic machinery that keeps cells alive. The new study adds a second, less appreciated role to that list: FAD is also a structural partner for the circadian clock, because it competes with the clock proteins cryptochrome 1 and 2 for binding to the ubiquitin E3 ligase, the cellular machine that tags proteins for destruction.

Here is where the chemistry becomes elegant. Cryptochromes are the workhorse repressors of the mammalian circadian oscillator, the proteins that help cells keep a roughly 24-hour rhythm of gene activity. Like all proteins, they are constantly being built and broken down, and the ubiquitin E3 ligase is responsible for their degradation. FAD competes with the cryptochrome proteins for access to that ligase, which means that when FAD levels are adequate, the degradation machinery is partially occupied and the cryptochromes survive longer, stabilizing the clock. When FAD runs short, the ligase turns its full attention to cryptochrome 1 and 2, and the clock proteins are chewed up faster than the cell can replace them.

The rat experiments showed precisely this sequence unfolding after cyclophosphamide exposure. Riboflavin kinase expression fell, riboflavin metabolism faltered, FAD synthesis was inhibited, and the cryptochrome proteins were degraded at an accelerated rate. The circadian rhythm regulation of the ovary, a tissue whose follicular development, hormone secretion and ovulation are all coordinated by daily molecular rhythms, fell into disarray. In other words, the chemotherapy drug was not only attacking the DNA of dividing granulosa cells; it was quietly dismantling the metabolic and temporal infrastructure those cells depend on to function.

To test whether the connection was causal rather than coincidental, the researchers turned to molecular experiments in which they knocked down riboflavin kinase directly. The results reinforced the model. Suppressing the enzyme lowered levels of FAD synthase, the downstream enzyme that completes FAD production, and it worsened the degradation of cryptochrome 1 and 2. Circadian rhythm regulation was disrupted, and the consequences rippled into the function of ovarian granulosa cells, the somatic cells that nurse developing follicles and produce the hormones essential for fertility. With their clock destabilized and their metabolism compromised, the granulosa cells declined, and ovarian aging accelerated.

The most clinically tantalizing result came from the rescue experiment. When the researchers supplemented the system with FAD, the cascade of damage was mitigated. Restoring the flavin cofactor eased the pressure on the cryptochrome proteins, helped re-establish circadian regulation and protected granulosa cell function. In the framework of the study, FAD supplementation acted as a counterweight to the riboflavin metabolism defect that cyclophosphamide had created, suggesting a potential strategy for shielding the ovary during chemo. The authors conclude that cyclophosphamide can impair ovarian function by causing dysregulation of riboflavin metabolism, which in turn drives abnormal circadian rhythms.

The implications extend beyond oncology. Premature ovarian insufficiency affects roughly one percent of women and carries consequences that reach far beyond fertility, including bone loss, cardiovascular risk and psychological distress. If a substantial share of chemotherapy-induced ovarian damage is mediated through a metabolic cofactor rather than through DNA strand breaks alone, then the protective options broaden considerably. Dietary and pharmacological interventions aimed at preserving riboflavin metabolism, or at stabilizing cryptochrome proteins directly, could in principle complement the existing approaches to fertility preservation, which currently rely on egg or embryo freezing and on temporary ovarian suppression with hormonal analogs during treatment.

The study also adds the ovary to a growing list of tissues whose health appears to depend on the integrity of their peripheral circadian clocks. Circadian biologists have shown that nearly every organ runs its own internal timing system, synchronized by the brain’s master clock but capable of autonomous oscillation, and that disrupting these local clocks impairs tissue function. The ovary is an especially rhythm-sensitive organ: follicle maturation, steroidogenesis and ovulation follow daily patterns, and granulosa cells express clock genes whose cycling is thought to coordinate these processes. A chemotherapeutic agent that destabilizes cryptochrome proteins in granulosa cells is therefore attacking the ovary at a level that standard genotoxicity models never captured.

As with any animal study, the road from rat ovaries to human clinics is long, and the findings, published open access with support from the National Natural Science Foundation of China, will need validation in human ovarian tissue and eventually in patients. The dosing, timing and safety of FAD supplementation alongside chemotherapy remain to be established, and it is far too early for patients undergoing cyclophosphamide treatment to draw practical conclusions. But the study reframes a familiar hazard in a new light. A drug long understood as a DNA-damaging agent now appears to sabotage the ovary through a second route, by starving it of a vitamin-derived cofactor that keeps its cellular clocks running, and that second route, unlike DNA damage itself, may be one medicine can actually repair.

Subject of Research: Mechanisms of cyclophosphamide-induced ovarian damage via riboflavin metabolism and circadian rhythm disruption

Article Title: Cyclophosphamide as a reproductive hazard: disrupted riboflavin metabolism and circadian rhythms drive ovarian decline

Article References: Peng, Y., Ma, Z., Hu, D., Wu, S., Shi, J., Chen, J., & Tan, Y. (2026). Cyclophosphamide as a reproductive hazard: disrupted riboflavin metabolism and circadian rhythms drive ovarian decline. Journal of Ovarian Research. https://doi.org/10.1186/s13048-026-02293-6

Image Credits: AI Generated

DOI: 10.1186/s13048-026-02293-6

Keywords: cyclophosphamide, ovarian aging, premature ovarian insufficiency, riboflavin kinase, FAD, cryptochrome, circadian rhythm, granulosa cells, chemotherapy, fertility, vitamin B2, ovary