Lithium Versus Quetiapine in Treatment Resistant Depression

Inclusion Criteria: 1. Under the care of a GP and/or adult mental health services 2. Current episode of depression meeting DSM-5 criteria for major depressive disorder (MDD) - single or recurrent episode 3.17-item HAM-D score ≥ 14 - this cut-off reflects a pragmatic minimum severity of depression as also chosen in comparable studies such as STAR\*D (Rush et al 2006, Trivedi et al 2006) 4.Any gender and aged 18 years or over 5.Meet criteria for treatment resistant depression (Fekadu et al., 2009a; Cleare et al., 2015): current episode has not responded to at least two antidepressants given for at least 6 weeks at minimum therapeutic dose defined as fluoxetine ≥20mg/day, paroxetine ≥20mg/day, sertraline ≥50mg/day, citalopram ≥20mg/day, escitalopram ≥10mg/day, venlafaxine ≥75mg/day, duloxetine ≥60 mg/day, mirtazapine ≥15mg/day, tricyclic antidepressant ≥125mg/day, and dosage as guided by the national Maudsley Prescribing Guidelines or BNF for any other antidepressant. Please note, relapse whilst on an antidepressant also counts as a failed treatment trial 6.Current antidepressant treatment has remained unchanged and at, or above, a therapeutic dose for ≥6 weeks 7.Provision of written, informed consent. Exclusion Criteria: 1. Diagnosis of bipolar disorder (defined as meeting DSM-5 criteria for bipolar 1 or bipolar 2) on the MINI 7.0 (as recommended treatments are different for bipolar depression) 2. Diagnosis of current psychosis (as recommended treatments are different for current psychosis - antidepressants plus antipsychotics is the first-line treatment recommendation (NiCE, 2009; Cleare et al., 2015) 3. Adequate use of lithium or quetiapine during the current episode. An adequate dose of lithium is defined as the patient taking lithium for at least 4 weeks at an adequate dose (leading to a documented plasma concentration of \>0.4mmol/L) and for quetiapine, prescription in the range of 150-300mg/d for 4 weeks or longer. Or, if the patient has taken an inadequate dose of lithium or quetiapine in the current episode, the patient and clinician are not willing to re-prescribe/take the medication. 4. Ongoing use of another atypical antipsychotic (discontinuation will be required before study entry i.e. any time prior to randomisation) 5. Known contraindication to use of either lithium or quetiapine: known hypersensitivity of lithium or quetiapine or any of their excipients; severe renal insufficiency / impairment; untreated hypothyroidism; severe cardiac disease / insufficiency; low sodium levels e.g. dehydrated patients or those on low sodium diets; Addison's disease; Brugada syndrome or family history of Brugada syndrome; the rare hereditary inborn errors of metabolism galactosaemia, the Lapp lactase deficiency or glucose-galactose malabsorption; concomitant administration of cytochrome P450 3A4 inhibitors; or congenital QT prolongation. 6. We will not recruit any individual who is currently participating in a clinical trial of an investigational medical product (CTIMP). 7. Insufficient degree of comprehension or attention to be able to engage in trial procedures. 8. We will exclude women who are pregnant, actively trying for pregnancy, or currently breastfeeding. This will be based on verbal report of the subject. Otherwise the management will be as appropriate according to standard clinical practice within the context of a pragmatic, open trial, for example adequate contraceptive precautions decided on the clinical judgement of the prescriber.