Zoladex and the invention of the monthly peptide depot — how AstraZeneca turned goserelin into an implant
Already with leuprolide in 1985 it was clear: GnRH analogs can pharmacologically 'castrate' prostate cancer. But daily injections were no comfort in an oncological patient population. In 1989 Imperial Chemical Industries (later AstraZeneca) launched Zoladex — goserelin in a biodegradable polymer matrix, in a pre-filled applicator, deposited subcutaneously, one implant per month. It was the first complete peptide depot.
The comfort problem of GnRH therapy
GnRH agonists (see the separate Schally article) have a pharmacological character requiring constant or at least frequent administration: with pulsatile or interrupted dosing, the action reverses from down-regulation to stimulation. Leuprolide (Lupron, approved 1985) was initially available as a daily subcutaneous injection. For a 70-year-old prostate cancer patient who must continue therapy for years, a daily injection is at least uncomfortable — and difficult to sustain over long periods.
The pharmacotechnological goal was clear: release the substance so that, after a single administration, constant therapeutic plasma levels are achieved over weeks or months. With small molecules the concept was already established (depot antipsychotics, depot contraceptives). With peptides it was technically more demanding because peptides are hydrophilic and often poorly soluble or unstable in polymer matrices.
The ICI solution: PLGA matrix in an applicator
Imperial Chemical Industries Pharma in Macclesfield (UK) developed from the mid-1980s a formulation enclosing the peptide goserelin (a synthetic GnRH analog with D-Ser(tBu)6 substitution for DPP-IV resistance) in a matrix of poly(D,L-lactide-co-glycolide) — PLGA for short. PLGA is a biodegradable polymer that decomposes through hydrolysis in skin over weeks, slowly releasing the embedded peptide. The matrix was formed into an approximately 1 mm thick, 1 cm long cylinder and offered in a pre-filled applicator with a 16-gauge needle — the patient received the injection subcutaneously into the abdominal wall, the implant remained there and released goserelin over about 28 days.
In 1989 Zoladex received approval in the UK for advanced prostate cancer, shortly afterwards in the US. A 3-month version (10.8 mg instead of 3.6 mg) followed in 1996. Clinically this was a comfort leap: 30 daily injections per month became one administration per month, later per quarter.
The successor depots: Lupron Depot, Trelstar, Eligard
Zoladex was the first peptide depot of this kind on the market, but not the only one. Lupron Depot (leuprolide in PLGA microspheres, suspension for intramuscular injection) was also approved in 1989. Trelstar (triptorelin, also microspheres) followed in 2000. Eligard (leuprolide in an in-situ-forming PLGA depot solution — injected in liquid form and forming a depot under the skin) was approved in 2002. Each of these depots uses a different pharmacotechnological variant of the same basic idea: PLGA-based sustained release.
PLGA technology was applied beyond the GnRH family from the 1990s — Octreotide-LAR (1998, monthly somatostatin depot), Risperidone-Consta (2003, antipsychotic depot), exenatide-Bydureon (2012, weekly GLP-1 depot). Thus a solution to a specific indication problem (GnRH comfort in prostate cancer) became a generic platform technology.
„The pharmacological problem was solved — we had an effective peptide. The actual problem was galenics: how do you get a hydrophilic molecule embedded in a hydrophobic matrix to dissolve at a constant rate over four weeks?"
What the Zoladex story galenically shows
The ICI/Zeneca/AstraZeneca solution was not a new active — goserelin was structurally related to leuprolide and other GnRH agonists in the Schally line. The innovation lay in galenics (pharmaceutical technology): the combination of the right polymer, right milling granulometry, right loading density and right applicator geometry. This kind of innovation is invisible in popular pharmacology discussion — people talk about substances, not formulations — but it makes the difference between a clinically usable medicine and a theoretically effective active.
The Zoladex story is also an economic story. Goserelin as an active has been patent-free since the 1990s, but the depot formulation remained patent-protected for a long time. To this day a generic for goserelin as powder for solution costs only a fraction of Zoladex; the depot formulation itself is the commercial substance, not the peptide. This split — cheap active, expensive galenics — is a pattern that recurs in many long-acting peptide lines.
Open questions
- Which new polymer matrices replace PLGA — are there alternatives with more precise release control and better biodegradability?
- How does the market change as depot patents expire — will generics manufacturers economically produce these complex formulations?
- What role do oral peptide formulations (Rybelsus for semaglutide since 2019) play as competition to depots — will subcutaneous implants be replaced by daily tablets?
- Which new indications outside oncology and endocrinology would benefit from peptide depots — chronic autoimmune diseases, hormone replacement?