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Nafarelin spray(synarel) is a potent gonadotropin-releasing hormone (GnRH) agonist for nasal administration. Featuring nasal mucosal absorption as its core advantage, it is more convenient and non-invasive than injectable formulations with superior patient compliance. The drug is rapidly absorbed through the nasal mucosa, reaching peak plasma concentration within 10–40 minutes and exerting pharmacological effects quickly. Its core mechanism lies in the biphasic regulation of the hypothalamic-pituitary-gonadal (HPG) axis.
This preparation is mainly indicated for the treatment of endometriosis and uterine fibroids, suppression of central precocious puberty in children, palliative treatment of prostate cancer, and pituitary down-regulation in assisted reproduction. With the advantages of non-invasive administration and definite therapeutic efficacy, it has become an important topical preparation for the treatment of reproductive system-related diseases.
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Biphasic Action Process
After entering the body, nafarelin spray does not produce an inhibitory effect directly but exhibits a typical biphasic action, divided into two consecutive progressive phases: the initial agonistic phase and the long-term down-regulation and desensitization phase, with significant differences in effects and mechanisms between the two stages.
The initial agonistic phase mainly occurs within the first 1–2 weeks of medication. As a potent gn-rh agonist, nafarelin specifically binds to gn-rh receptors on gonadotropic cells of the anterior pituitary, transiently activating receptor signaling pathways and stimulating the pituitary to secrete large amounts of luteinizing hormone (LH) and follicle-stimulating hormone (FSH).
The elevated secretion of LH and FSH further acts on the gonads (ovaries or testes), causing transient hyperfunction of gonadal activity and a temporary rise in estrogen levels in females and testosterone levels in males.
This short-term fluctuation in hormone levels may transiently aggravate original symptoms in some patients: female patients may experience worsened dysmenorrhea and temporary ovarian enlargement, while male patients may suffer testicular distending pain and transient increased libido. Symptoms in this phase are usually temporary and gradually relieved with prolonged medication without special intervention. However, patients should be informed in advance to avoid unnecessary anxiety.
The long-term down-regulation and desensitization phase generally emerges after 2–4 weeks of continuous medication and serves as the key stage for synarel to exert therapeutic effects. Owing to its high affinity and prolonged action on gn-rh receptors, persistent overstimulation triggers adaptive changes in gn-rh receptors on anterior pituitary gonadotropic cells, manifested as receptor desensitization and receptor internalization down-regulation.
Receptor desensitization refers to a marked decline in receptor sensitivity to synarel and endogenous gn-rh, failing to initiate effective signal transduction even after binding.
Receptor internalization down-regulation means gn-rh receptors on the cell membrane are internalized into cells, drastically reducing the number of available binding receptors on the membrane. The combined effect of these two changes significantly inhibits basal secretion of LH and FSH, lowering their levels to an extremely low range.
With the secretion of LH and FSH suppressed, the gonads lose gonadotropin stimulation and their function is gradually inhibited, ultimately leading to medical castration or pseudomenopause.
In female patients, ovarian estradiol secretion drops to postmenopausal levels, causing endometrial atrophy due to the loss of estrogen support. In male patients, testicular testosterone secretion decreases to castration levels, thereby reducing the androgenic effect on target organs.
This drug-induced suppression of gonadal function is reversible. Upon drug discontinuation, pituitary gn-rh receptor function gradually recovers, LH and FSH secretion returns to normal, and gonadal function is progressively restored to the pre-medication state.
Information Sources: MIMS Hong Kong Synarel: Uses, Dosage, Side Effects and More; Drugs.com Synarel (Monograph).
Core Pharmacological Effects
Based on the above biphasic mechanism, nafarelin spray inhibits HPG axis function and reduces sex hormone levels to produce multiple clinically relevant pharmacological effects, widely applied in the treatment of reproductive system diseases and related fields.
Inhibit proliferation of endometriotic lesionsThe core pathological mechanism of endometriosis is the abnormal proliferation and bleeding of ectopic endometrial tissue stimulated by estrogen, triggering pain, infertility and other symptoms.
By long-term suppression of estrogen secretion, synarel deprives ectopic endometrial tissue of estrogen nutritional support, leading to gradual atrophy and regression of lesions, thereby reducing lesion volume, alleviating pain and improving patients' quality of life - one of its most important clinical applications.
Shrink uterine fibroidsUterine fibroids are common benign tumors of the female reproductive system, whose growth is dependent on estrogen stimulation; elevated estrogen levels promote fibroid proliferation and enlargement. By reducing estradiol levels, synarel inhibits the proliferation of uterine fibroid cells and induces their apoptosis, thus shrinking fibroid volume.
It is especially suitable for patients with small fibroids who refuse surgical treatment, or used as preoperative adjuvant therapy to reduce fibroid volume and lower surgical difficulty.Suppress precocious puberty development in childrenCentral precocious puberty (true precocious puberty) in children is mainly caused by premature activation of the HPG axis, leading to early excessive secretion of LH, FSH and sex hormones and premature emergence of secondary sexual characteristics accompanied by accelerated growth and development.
By long-term down-regulation of pituitary gn-rh receptors and inhibiting LH and FSH secretion, synarel reduces sex hormone levels to prepubertal status, thereby halting further development of secondary sexual characteristics, delaying skeletal maturation and preventing short final adult height caused by premature development. It is a crucial drug for the treatment of pediatric central precocious puberty.
Cut off androgen supply for prostate cancerThe occurrence and progression of prostate cancer are highly dependent on androgen stimulation, and reducing androgen supply is one of the key therapeutic strategies.
By inhibiting pituitary LH secretion, synarel reduces testicular testosterone synthesis and secretion, lowering testosterone to castration levels, thereby cutting off the androgen source of prostate cancer cells, inhibiting cancer cell proliferation and metastasis, and achieving palliative treatment of prostate cancer.Pituitary down-regulation and prevention of premature LH surge in assisted reproductionIn assisted reproductive technologies such as in vitro fertilization and embryo transfer, premature pituitary LH secretion (premature LH surge) causes premature oocyte maturation, impairing oocyte quality and fertilization success rate.
By suppressing pituitary function to achieve pituitary down-regulation, synarel effectively prevents premature LH surge, synchronizes follicular development, facilitates physicians to control oocyte retrieval time, and improves the success rate of assisted reproduction.
In addition, after nasal administration, nafarelin spray is rapidly absorbed through the nasal mucosa into the blood circulation. In adult females, peak plasma concentration is reached within 10–40 minutes after nasal dosing, with an average bioavailability of 2.8%, plasma protein binding rate of approximately 80%, and a half-life of about 3 hours. The drug is mainly excreted via urine and feces, and its metabolites show no significant pharmacological activity. Its pharmacokinetic properties ensure the timeliness and safety of drug action.
Information Sources: Pfizer Medical Clinical Pharmacology; Drugs.com Nafarelin (Monograph); Tocris Bioscience Nafarelin Product Datasheets.
The mainstream industrial synthetic route consists of three key steps:
First, the N-terminal pentapeptide fragment (Boc-Pyr-His(Boc)-Trp(Boc)-Ser(tBu)-Tyr(tBu)-OH) is synthesized by solid-phase peptide synthesis (SPPS). Using CTC resin as the solid support and appropriate amino acid monomers, amino acids are sequentially coupled from the C-terminus to the N-terminus via condensing agent-mediated coupling reactions. High-purity pentapeptide fragments are obtained after deprotection and cleavage.
Second, the intermediate dipeptide fragment (D-2-Nal-Leu-OMe) and the C-terminal tripeptide fragment (H-Arg(pbf)-Pro-NHEt) are separately synthesized by liquid-phase synthesis. Condensing agents and activators are used to facilitate amino acid coupling, and target fragments are acquired through deprotection reactions.
Finally, the above three fragments are sequentially coupled in the liquid phase to form a complete decapeptide chain. After side-chain protecting group removal, concentration, precipitation, centrifugation and drying, crude synarel is obtained. Further purification via reversed-phase high-performance liquid chromatography (RP-HPLC) yields high-purity synarel with purity ≥90%, which is ultimately prepared into acetate salt for nasal spray formulation.
In the synthesis process, the selection of condensing agents (e.g., HBTU, DIC), rational application of protecting groups (e.g., Boc, tBu, pbf), and control of reaction conditions (temperature, pH value, reaction time) are critical to ensuring synthesis efficiency and product purity. Protecting groups prevent side reactions of amino acid side chains during coupling; after peptide chain synthesis, they are removed with specific reagents without affecting the normal structure and pharmacological activity of the peptide chain.
The combined solid-liquid synthesis method effectively overcomes the drawbacks of insufficient purity of pure solid-phase synthesis and cumbersome procedures of pure liquid-phase synthesis, improves synthetic yield and product quality, and is suitable for large-scale industrial production.
Information Sources: Shanghai Kaiyin Chemical Co., Ltd. Synarel; X Platform A Solid-Liquid Synthesis Method and Process of Synarel; Drugs.com Nafarelin (Monograph).
Psychiatric and Neurological Risks
It may induce or exacerbate depression, anxiety and mood swings. Close monitoring is required for patients with a history of psychiatric disorders.
Rare cases of epileptic seizures and intracranial hypertension (pseudotumor cerebri) have been reported; the risk is higher in patients with a history of epilepsy or cerebrovascular diseases, or those concomitantly using antidepressants such as SSRIs.
Immediate drug discontinuation and medical consultation are mandatory if persistent low mood, headache or visual abnormalities occur during medication.
Information Sources: Pfizer Prescribing Information (SYNAREL- nafarelin acetate spray, metered), 2025; FDA Prescribing Information, 2025.
FAQ
Does it stop periods?
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During the time you are using nafarelin, your menstrual period may not be regular or you may not have a menstrual period at all. This is to be expected when being treated with this medicine. If regular menstruation does not begin within 2 to 3 months after you stop using this medicine, check with your doctor.
Can you get pregnant on Synarel?
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If you miss successive doses of Synarel (two or more doses following each other), have had sexual intercourse and have not been using contraceptive measures (as described in the section "What should I know while using Synarel?"), there is a possibility you could become pregnant.
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